virgl: Avoid unfinished transfer_get with PIPE_TRANSFER_DONTBLOCK

virgl: Avoid unfinished transfer_get with PIPE_TRANSFER_DONTBLOCK

If we are not allowed to block, and we know that we will have to wait,
either because the resource is busy, or because it will become busy due
to a readback, return early to avoid performing an incomplete
transfer_get. Such an incomplete transfer_get may finish at any time,
during which another unsynchronized map could write to the resource
contents, leaving the contents in an undefined state.

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