drm/cirrus: NULL-check pipe->plane.state->fb in cirrus_pipe_update()

After having been compared to NULL value at cirrus.c:455, pointer
'pipe->plane.state->fb' is passed as 1st parameter in call to function
'cirrus_fb_blit_rect' at cirrus.c:461, where it is dereferenced at
cirrus.c:316.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

v2:
	* aligned commit message to line-length limits
Signed-off-by: Alexandr Sapozhnikov <alsp705@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215171549.16305-1-alsp705@gmail.com

drivers/gpu/drm/tiny/cirrus.c

@@ -455,7 +455,7 @@ static void cirrus_pipe_update(struct drm_simple_display_pipe *pipe,
 	if (state->fb && cirrus->cpp != cirrus_cpp(state->fb))
 		cirrus_mode_set(cirrus, &crtc->mode, state->fb);
 
-	if (drm_atomic_helper_damage_merged(old_state, state, &rect))
+	if (state->fb && drm_atomic_helper_damage_merged(old_state, state, &rect))
 		cirrus_fb_blit_rect(state->fb, &shadow_plane_state->data[0], &rect);
 }