drm/virtio: factor out the sg_table from virtio_gpu_object

A resource will be a shmem based resource or a (planned)
vram based resource, so it makes sense to factor out common fields
(resource handle, dumb).

v2: move mapped field to shmem object

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200305013212.130640-1-gurchetansingh@chromium.org


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