drm/fb-helper: generic: Fix drm_fbdev_client_restore()

drm/fb-helper: generic: Fix drm_fbdev_client_restore()

If fbdev setup has failed, lastclose will give a NULL pointer deref:

[   77.794295] [drm:drm_lastclose]
[   77.794414] [drm:drm_lastclose] driver lastclose completed
[   77.794660] Unable to handle kernel NULL pointer dereference at virtual address 00000014
[   77.809460] pgd = b376b71b
[   77.818275] [00000014] *pgd=175ba831, *pte=00000000, *ppte=00000000
[   77.830813] Internal error: Oops: 17 [#1] ARM
[   77.840963] Modules linked in: mi0283qt mipi_dbi tinydrm raspberrypi_hwmon gpio_backlight backlight snd_bcm2835(C) bcm2835_rng rng_core
[   77.865203] CPU: 0 PID: 527 Comm: lt-modetest Tainted: G         C        5.0.0-rc1+ #1
[   77.879525] Hardware name: BCM2835
[   77.889185] PC is at restore_fbdev_mode+0x20/0x164
[   77.900261] LR is at drm_fb_helper_restore_fbdev_mode_unlocked+0x54/0x9c
[   78.002446] Process lt-modetest (pid: 527, stack limit = 0x7a3d5c14)
[   78.291030] Backtrace:
[   78.300815] [<c04f2d0c>] (restore_fbdev_mode) from [<c04f4708>] (drm_fb_helper_restore_fbdev_mode_unlocked+0x54/0x9c)
[   78.319095]  r9:d8a8a288 r8:d891acf0 r7:d7697910 r6:00000000 r5:d891ac00 r4:d891ac00
[   78.334432] [<c04f46b4>] (drm_fb_helper_restore_fbdev_mode_unlocked) from [<c04f47e8>] (drm_fbdev_client_restore+0x18/0x20)
[   78.353296]  r8:d76978c0 r7:d7697910 r6:d7697950 r5:d7697800 r4:d891ac00 r3:c04f47d0
[   78.368689] [<c04f47d0>] (drm_fbdev_client_restore) from [<c051b6b4>] (drm_client_dev_restore+0x7c/0xc0)
[   78.385982] [<c051b638>] (drm_client_dev_restore) from [<c04f8fd0>] (drm_lastclose+0xc4/0xd4)
[   78.402332]  r8:d76978c0 r7:d7471080 r6:c0e0c088 r5:d8a85e00 r4:d7697800
[   78.416688] [<c04f8f0c>] (drm_lastclose) from [<c04f9088>] (drm_release+0xa8/0x10c)
[   78.431929]  r5:d8a85e00 r4:d7697800
[   78.442989] [<c04f8fe0>] (drm_release) from [<c02640c4>] (__fput+0x104/0x1c8)
[   78.457740]  r8:d5ccea10 r7:d96cfb10 r6:00000008 r5:d74c1b90 r4:d8a8a280
[   78.472043] [<c0263fc0>] (__fput) from [<c02641ec>] (____fput+0x18/0x1c)
[   78.486363]  r10:00000006 r9:d7722000 r8:c01011c4 r7:00000000 r6:c0ebac6c r5:d892a340
[   78.501869]  r4:d8a8a280
[   78.512002] [<c02641d4>] (____fput) from [<c013ef1c>] (task_work_run+0x98/0xac)
[   78.527186] [<c013ee84>] (task_work_run) from [<c010cc54>] (do_work_pending+0x4f8/0x570)
[   78.543238]  r7:d7722030 r6:00000004 r5:d7723fb0 r4:00000000
[   78.556825] [<c010c75c>] (do_work_pending) from [<c0101034>] (slow_work_pending+0xc/0x20)
[   78.674256] ---[ end trace 70d3a60cf739be3b ]---

Fix by using drm_fb_helper_lastclose() which checks if fbdev is in use.

Fixes: 9060d7f49376 ("drm/fb-helper: Finish the generic fbdev emulation")
Cc: stable@vger.kernel.org
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125150300.33268-1-noralf@tronnes.org