drm/udl: limit modes to the sku pixel limits.

Otherwise when X starts we commonly get a black screen scanning
out nothing, its wierd dpms on/off from userspace brings it back,

With this on F18, multi-seat works again with my 1920x1200 monitor
which is above the sku limit for the device I have.

Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexander.deucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/udl/udl_connector.c

@@ -69,6 +69,13 @@ static int udl_get_modes(struct drm_connector *connector)
 static int udl_mode_valid(struct drm_connector *connector,
 			  struct drm_display_mode *mode)
 {
+	struct udl_device *udl = connector->dev->dev_private;
+	if (!udl->sku_pixel_limit)
+		return 0;
+
+	if (mode->vdisplay * mode->hdisplay > udl->sku_pixel_limit)
+		return MODE_VIRTUAL_Y;
+
 	return 0;
 }