drm/mgag200: 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/20180424131508.2210-1-luc.vanoostenryck@gmail.com

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -1586,7 +1586,7 @@ static uint32_t mga_vga_calculate_mode_bandwidth(struct drm_display_mode *mode,
 
 #define MODE_BANDWIDTH	MODE_BAD
 
-static int mga_vga_mode_valid(struct drm_connector *connector,
+static enum drm_mode_status mga_vga_mode_valid(struct drm_connector *connector,
 				 struct drm_display_mode *mode)
 {
 	struct drm_device *dev = connector->dev;