virgl: quadruple command buffer size

Tested running WebGL aquarium on Nvidia host (10,000 fishes)

This moves us from 7 fps to 9 fps.  After quadrupling, performance
gains diminish.

v2: Remove change ID (Erik)

Tested-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
(cherry picked from commit c0773315)