virgl: add virgl_resource_transfer_prepare

virgl: add virgl_resource_transfer_prepare

virgl_resource_transfer_prepare should be called before mapping to
prepare the resource.  It does flush, readback, and wait as needed.
virgl_res_needs_flush and virgl_res_needs_readback become internal
helpers to the new function.

There should be no externally visible change.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>