virgl: make winsys modifications for encoded transfers

virgl: make winsys modifications for encoded transfers

The idea is to have two command buffers:

1) One for transfers
2) One for commands, which can include transfers

At flush time, (2) will be filled.  Otherwise, (1) will be
used to submit transfers if there are enough of them.

v2: Pass size directly to cmd_buf_create (@gerddie)
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>