drm: qxl: add missing return check

My static checker complains that "release" is uninitialized if
qxl_alloc_release_reserved() fails, so let's add a check for that.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170314075410.GB5984@mwandaSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/qxl/qxl_display.c

@@ -572,6 +572,8 @@ static void qxl_cursor_atomic_update(struct drm_plane *plane,
 	ret = qxl_alloc_release_reserved(qdev, sizeof(*cmd),
 					 QXL_RELEASE_CURSOR_CMD,
 					 &release, NULL);
+	if (ret)
+		return;
 
 	cmd = (struct qxl_cursor_cmd *) qxl_release_map(qdev, release);