drm/amd/display: Drop 'acrtc' and add 'new_crtc_state' NULL check for writeback requests.

drm/amd/display: Drop 'acrtc' and add 'new_crtc_state' NULL check for writeback requests.

Return value of 'to_amdgpu_crtc' which is container_of(...) can't be
null, so it's null check 'acrtc' is dropped.

Fixing the below:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9302 amdgpu_dm_atomic_commit_tail() error: we previously assumed 'acrtc' could be null (see line 9299)

Added 'new_crtc_state' NULL check for function
'drm_atomic_get_new_crtc_state' that retrieves the new state for a CRTC,
while enabling writeback requests.

Cc: [email protected]
Cc: Alex Hung <[email protected]>
Cc: Aurabindo Pillai <[email protected]>
Cc: Rodrigo Siqueira <[email protected]>
Cc: Hamza Mahfooz <[email protected]>
Signed-off-by: Srinivasan Shanmugam <[email protected]>
Reviewed-by: Alex Hung <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>