drm/plane-helper: Test for plane disable earlier

drm_plane_helper_check_update() currently uses crtc before testing whether
we're disabling the plane (fb == NULL).  Move the fb test before the first crtc
usage so that crtc == NULL doesn't have to be handled by the caller.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_plane_helper.c

@@ -142,6 +142,17 @@ int drm_plane_helper_check_update(struct drm_plane *plane,
 {
 	int hscale, vscale;
 
+	if (!fb) {
+		*visible = false;
+		return 0;
+	}
+
+	/* crtc should only be NULL when disabling (i.e., !fb) */
+	if (WARN_ON(!crtc)) {
+		*visible = false;
+		return 0;
+	}
+
 	if (!crtc->enabled && !can_update_disabled) {
 		DRM_DEBUG_KMS("Cannot update plane of a disabled CRTC.\n");
 		return -EINVAL;
@@ -155,11 +166,6 @@ int drm_plane_helper_check_update(struct drm_plane *plane,
 		return -ERANGE;
 	}
 
-	if (!fb) {
-		*visible = false;
-		return 0;
-	}
-
 	*visible = drm_rect_clip_scaled(src, dest, clip, hscale, vscale);
 	if (!*visible)
 		/*