drm/rcar-du: Fix error check when retrieving crtc state

drm_atomic_get_crtc_state() returns an error pointer when it fails, so
the null check is doing nothing here.

Credit to 0-day/Dan Carpenter for reporting this.

Fixes: 6f3b6278 ("drm: Convert connector_helper_funcs->atomic_check to accept drm_atomic_state")
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com> [for rcar lvds]
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Karol Herbst <karolherbst@gmail.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: linux-renesas-soc@vger.kernel.org
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190617181548.124134-1-sean@poorly.run

drivers/gpu/drm/rcar-du/rcar_lvds.c

@@ -115,8 +115,8 @@ static int rcar_lvds_connector_atomic_check(struct drm_connector *connector,
 
 	/* We're not allowed to modify the resolution. */
 	crtc_state = drm_atomic_get_crtc_state(state, conn_state->crtc);
-	if (!crtc_state)
-		return -EINVAL;
+	if (IS_ERR(crtc_state))
+		return PTR_ERR(crtc_state);
 
 	if (crtc_state->mode.hdisplay != panel_mode->hdisplay ||
 	    crtc_state->mode.vdisplay != panel_mode->vdisplay)