drm/virtio: Call the right shmem helpers

drm_gem_shmem_get_sg_table is meant to implement
obj->funcs->get_sg_table, for prime exporting. The one we want is
drm_gem_shmem_get_pages_sgt, which also handles imported dma-buf, not
just native objects.

v2: Rebase, this stuff moved around in

commit 2f2aa137
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Fri Feb 7 08:46:38 2020 +0100

    drm/virtio: move virtio_gpu_mem_entry initialization to new function

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200511093554.211493-5-daniel.vetter@ffwll.ch