drm/virtio: switch from ttm to gem shmem helpers

virtio-gpu basically needs a sg_table for the bo, to tell the host where
the backing pages for the object are.  So the gem shmem helpers are a
perfect fit.  Some drm_gem_object_funcs need thin wrappers to update the
host state, but otherwise the helpers handle everything just fine.

Once the fencing was sorted the switch was surprisingly easy and for the
most part just removing the ttm code.

v4: fix drm_gem_object_funcs name.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-15-kraxel@redhat.com