drm/amdgpu/uvd7: add emit_reg_write_reg_wait ring callback

drm/amdgpu/uvd7: add emit_reg_write_reg_wait ring callback

Fix the NULL pointer dereference while running amdgpu_test:

[   54.972246] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
[   54.972265] IP:           (null)
[   54.972273] PGD 0 P4D 0
[   54.972280] Oops: 0010 [#1] SMP PTI
[   54.972288] Modules linked in: amdkfd amd_iommu_v2 amdgpu(OE) chash gpu_sched ttm drm_kms_helper drm i2c_algo_bit fb_sys_fops syscopyarea sysfillrect sysimgblt snd_hda_codec_realtek snd_hda_codec_generic snd_hda_codec_hdmi snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep intel_rapl snd_pcm snd_seq_midi snd_seq_midi_event snd_rawmidi x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel snd_seq snd_seq_device kvm irqbypass snd_timer crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc snd soundcore joydev input_leds aesni_intel aes_x86_64 crypto_simd glue_helper cryptd idma64 virt_dma mei_me intel_lpss_pci serio_raw intel_cstate intel_rapl_perf shpchp intel_pch_thermal mei mac_hid intel_lpss acpi_pad parport_pc ppdev nfsd lp auth_rpcgss nfs_acl lockd grace sunrpc parport autofs4 hid_generic
[   54.972434]  usbhid mxm_wmi e1000e psmouse ahci hid libahci wmi pinctrl_sunrisepoint video pinctrl_intel
[   54.972457] CPU: 6 PID: 1393 Comm: uvd Tainted: G           OE    4.16.0-rc7-27fb84fda777 #1
[   54.972473] Hardware name: MSI MS-7984/Z170 KRAIT GAMING (MS-7984), BIOS B.80 05/11/2016
[   54.972489] RIP: 0010:          (null)
[   54.972497] RSP: 0018:ffffaea002c8bcc0 EFLAGS: 00010202
[   54.972508] RAX: 0000000000000000 RBX: ffff9d30d3c56f60 RCX: 00000000007c0002
[   54.972522] RDX: 000000000001a6fb RSI: 000000000001a6e9 RDI: ffff9d30d3c56f60
[   54.972536] RBP: ffffaea002c8bd10 R08: 0000000000000002 R09: ffffffffc06977d0
[   54.972550] R10: 0000000000000040 R11: 0000000000000000 R12: 0000000000000002
[   54.972564] R13: ffff9d30d3c5001c R14: ffff9d30d3c50000 R15: 0000000000000006
[   54.972579] FS:  0000000000000000(0000) GS:ffff9d30eed80000(0000) knlGS:0000000000000000
[   54.972594] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   54.972606] CR2: 0000000000000000 CR3: 00000002dbc0a001 CR4: 00000000003606e0
[   54.972620] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   54.972634] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   54.972648] Call Trace:
[   54.972685]  ? gmc_v9_0_emit_flush_gpu_tlb+0x111/0x140 [amdgpu]
[   54.972721]  uvd_v7_0_ring_emit_vm_flush+0x31/0x70 [amdgpu]
[   54.972751]  amdgpu_vm_flush+0x5dc/0x6c0 [amdgpu]
[   54.972787]  ? pp_dpm_powergate_uvd+0x50/0x80 [amdgpu]
[   54.972816]  amdgpu_ib_schedule+0x120/0x4e0 [amdgpu]
[   54.972850]  amdgpu_job_run+0x17b/0x1c0 [amdgpu]
[   54.972861]  drm_sched_main+0x2cc/0x490 [gpu_sched]
[   54.972873]  ? wait_woken+0x80/0x80
[   54.972882]  kthread+0x121/0x140
[   54.972891]  ? drm_sched_job_finish+0xf0/0xf0 [gpu_sched]
[   54.972902]  ? kthread_create_worker_on_cpu+0x70/0x70
[   54.972914]  ret_from_fork+0x35/0x40
[   54.972922] Code:  Bad RIP value.
[   54.972932] RIP:           (null) RSP: ffffaea002c8bcc0
[   54.972943] CR2: 0000000000000000
[   54.972951] ---[ end trace 5feb349263bbf633 ]---

Signed-off-by: Xiaojie Yuan <Xiaojie.Yuan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>