drm/panfrost: Fix sleeping while atomic in panfrost_gem_open

drm/panfrost: Fix sleeping while atomic in panfrost_gem_open

We can't hold the mm_lock spinlock as panfrost_mmu_map() can sleep:

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:909
in_atomic(): 1, irqs_disabled(): 0, pid: 974, name: glmark2-es2-drm
1 lock held by glmark2-es2-drm/974:
CPU: 5 PID: 974 Comm: glmark2-es2-drm Tainted: G        W    L    5.3.0-rc1+ #94
Hardware name: 96boards Rock960 (DT)
Call trace:
 dump_backtrace+0x0/0x130
 show_stack+0x14/0x20
 dump_stack+0xc4/0x10c
 ___might_sleep+0x158/0x228
 __might_sleep+0x50/0x88
 __mutex_lock+0x58/0x800
 mutex_lock_interruptible_nested+0x1c/0x28
 drm_gem_shmem_get_pages+0x24/0xa0
 drm_gem_shmem_get_pages_sgt+0x48/0xd0
 panfrost_mmu_map+0x38/0xf8 [panfrost]
 panfrost_gem_open+0xc0/0xd8 [panfrost]
 drm_gem_handle_create_tail+0xe8/0x198
 drm_gem_handle_create+0x3c/0x50
 panfrost_gem_create_with_handle+0x70/0xa0 [panfrost]
 panfrost_ioctl_create_bo+0x48/0x80 [panfrost]
 drm_ioctl_kernel+0xb8/0x110
 drm_ioctl+0x244/0x3f0
 do_vfs_ioctl+0xbc/0x910
 ksys_ioctl+0x78/0xa8
 __arm64_sys_ioctl+0x1c/0x28
 el0_svc_common.constprop.0+0x90/0x168
 el0_svc_handler+0x28/0x78
 el0_svc+0x8/0xc

Fixes: a5efb4c9a562 ("drm/panfrost: Restructure the GEM object creation")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819161204.3106-5-robh@kernel.org