glsl: add driconf to zero-init unintialized vars

glsl: add driconf to zero-init unintialized vars

Some games are sloppy.. perhaps because it is defined behavior for DX or
perhaps because nv blob driver defaults things to zero.

So add driconf param to force uninitialized variables to default to zero.

This issue was observed with rust, from steam store.  But has surfaced
elsewhere in the past.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>