drm/virtio: fix the missed drm_gem_object_put() in virtio_gpu_user_framebuffer_create()

virtio_gpu_user_framebuffer_create() misses to call drm_gem_object_put()
in an error path. Add the missed function call to fix it.

Signed-off-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1633770560-11658-1-git-send-email-jingxiangfeng@huawei.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>