drm/gma500: add a missed gma_power_end in error path

oaktrail_lvds_mode_set() misses a gma_power_end() in an error path.
Add the call to fix it.
Signed-off-by: NChuhong Yuan <hslester96@gmail.com>
Signed-off-by: NPatrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209085747.16057-1-hslester96@gmail.com

drivers/gpu/drm/gma500/oaktrail_lvds.c

@@ -117,6 +117,7 @@ static void oaktrail_lvds_mode_set(struct drm_encoder *encoder,
 
 	if (!connector) {
 		DRM_ERROR("Couldn't find connector when setting mode");
+		gma_power_end(dev);
 		return;
 	}