drm/virtio: Prepare resource_flush to accept a fence

A fence will be added to resource_flush for resources that
are guest blobs.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210610232456.671905-3-vivek.kasireddy@intel.com


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