virgl: handle DONT_BLOCK and MAP_DIRECTLY

virgl: handle DONT_BLOCK and MAP_DIRECTLY

Handle PIPE_TRANSFER_DONT_BLOCK and PIPE_TRANSFER_MAP_DIRECTLY.
Make virgl_resource_transfer_prepare return an enum instead of a
bool for extensibility (e.g., instruct the callers to map
differently).

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