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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -5671,7 +5671,8 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
 	}
 
 	/* Update the planes if changed or disable if we don't have any. */
-	if (planes_count || acrtc_state->active_planes == 0) {
+	if ((planes_count || acrtc_state->active_planes == 0) &&
+		acrtc_state->stream) {
 		if (new_pcrtc_state->mode_changed) {
 			bundle->stream_update.src = acrtc_state->stream->src;
 			bundle->stream_update.dst = acrtc_state->stream->dst;