drm/virtio: Correct drm_gem_shmem_get_sg_table() error handling

drm_gem_shmem_get_sg_table() never ever returned NULL on error. Correct
the error handling to avoid crash on OOM.

Cc: stable@vger.kernel.org
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>