drm/zte: plane_state->fb iff plane_state->crtc

Checking both is one too much, so wrap a WARN_ON around it to stope
the copypasta.
Reviewed-by: NRodrigo Siqueira <rodrigosiqueira@gmail.com>
Tested-by: NRodrigo Siqueira <rodrigosiqueira@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191213172612.1514842-10-daniel.vetter@ffwll.ch

drivers/gpu/drm/zte/zx_plane.c

@@ -54,7 +54,7 @@ static int zx_vl_plane_atomic_check(struct drm_plane *plane,
 	int min_scale = FRAC_16_16(1, 8);
 	int max_scale = FRAC_16_16(8, 1);
 
-	if (!crtc || !fb)
+	if (!crtc || WARN_ON(!fb))
 		return 0;
 
 	crtc_state = drm_atomic_get_existing_crtc_state(plane_state->state,
@@ -281,7 +281,7 @@ static int zx_gl_plane_atomic_check(struct drm_plane *plane,
 	struct drm_crtc *crtc = plane_state->crtc;
 	struct drm_crtc_state *crtc_state;
 
-	if (!crtc || !fb)
+	if (!crtc || WARN_ON(!fb))
 		return 0;
 
 	crtc_state = drm_atomic_get_existing_crtc_state(plane_state->state,