drm/msm: atomic fixes

Fixes for a couple little issues found in testing.

Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/gpu/drm/msm/msm_atomic.c b/drivers/gpu/drm/msm/msm_atomic.c
index 8cf3361..f0de412 100644
--- a/drivers/gpu/drm/msm/msm_atomic.c
+++ b/drivers/gpu/drm/msm/msm_atomic.c
@@ -117,7 +117,7 @@
 		if (!plane)
 			continue;
 
-		if (plane->state->fb != new_state->fb)
+		if ((plane->state->fb != new_state->fb) && new_state->fb)
 			add_fb(c, new_state->fb);
 	}