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_fb.c b/drivers/gpu/drm/msm/msm_fb.c
index f4e42d5..84dec16 100644
--- a/drivers/gpu/drm/msm/msm_fb.c
+++ b/drivers/gpu/drm/msm/msm_fb.c
@@ -120,6 +120,8 @@
 uint32_t msm_framebuffer_iova(struct drm_framebuffer *fb, int id, int plane)
 {
 	struct msm_framebuffer *msm_fb = to_msm_framebuffer(fb);
+	if (!msm_fb->planes[plane])
+		return 0;
 	return msm_gem_iova(msm_fb->planes[plane], id);
 }