virgl: Add driconf tweak for emulating BGRA surfaces on GLES

virgl: Add driconf tweak for emulating BGRA surfaces on GLES

These tweaks are used to fix rendering issues with Valve games and
at least also "The Raven Remastered" when run on a GLES host.

v2: Fix type in define and remove virgl from driconf option (Emil)
v3: Encode tweak binary instead of using strings (Gurchetan)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>