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: stable@vger.kernel.org
Cc: Alex Hung <alex.hung@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>