drm/amd/amdgpu: Check stream in amdgpu_dm_commit_planes

Reported by smatch:
amdgpu_dm.c:5637 amdgpu_dm_commit_planes() error: we previously assumed 'acrtc_state->stream' could be null
This seems to be checked for null pretty consistently elsewhere.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>