drm/virtio: fix mode_valid's return type

The method struct drm_connector_helper_funcs::mode_valid is defined
as returning an 'enum drm_mode_status' but the driver implementation
for this method uses an 'int' for it.

Fix this by using 'enum drm_mode_status' in the driver too.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180424131524.2510-1-luc.vanoostenryck@gmail.com
Link: https://patchwork.freedesktop.org/patch/msgid/20180424131515.2360-1-luc.vanoostenryck@gmail.com
Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org

drivers/gpu/drm/virtio/virtgpu_display.c

@@ -208,7 +208,7 @@ static int virtio_gpu_conn_get_modes(struct drm_connector *connector)
 	return count;
 }
 
-static int virtio_gpu_conn_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status virtio_gpu_conn_mode_valid(struct drm_connector *connector,
 				      struct drm_display_mode *mode)
 {
 	struct virtio_gpu_output *output =