virgl: save virgl_hw_res in virgl_transfer

virgl: save virgl_hw_res in virgl_transfer

When PIPE_TRANSFER_DISCARD_WHOLE_RESOURCE is properly supported,
virgl_transfer might refer to a different virgl_hw_res than
virgl_resource does.  We need to save the virgl_hw_res and use the
saved one.

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