drm/xen: Simplify fb_create

The current code is a pretty good wtf moment, since we drop the
reference before we use it. It's not a big deal, because a) we only
use the pointer, so doesn't blow up and the real reason b) fb->obj[0]
already holds a full reference for us.

Might as well take the real pointer ins't of complicated games that
baffle.
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Cc: xen-devel@lists.xenproject.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191115092120.4445-7-daniel.vetter@ffwll.ch

drivers/gpu/drm/xen/xen_drm_front_kms.c

@@ -63,14 +63,7 @@ fb_create(struct drm_device *dev, struct drm_file *filp,
 	if (IS_ERR_OR_NULL(fb))
 		return fb;
 
-	gem_obj = drm_gem_object_lookup(filp, mode_cmd->handles[0]);
-	if (!gem_obj) {
-		DRM_ERROR("Failed to lookup GEM object\n");
-		ret = -ENOENT;
-		goto fail;
-	}
-
-	drm_gem_object_put_unlocked(gem_obj);
+	gem_obj = fb->obj[0];
 
 	ret = xen_drm_front_fb_attach(drm_info->front_info,
 				      xen_drm_front_dbuf_to_cookie(gem_obj),