i965: Add a brw->scalar_gs flag controlled by INTEL_SCALAR_GS=1.

i965: Add a brw->scalar_gs flag controlled by INTEL_SCALAR_GS=1.

This patch introduces a brw->scalar_gs flag, similar to brw->scalar_vs,
which controls whether or not to use SIMD8 geometry shaders.

For now, we control it via a new environment variable, INTEL_SCALAR_GS.
This provides a convenient way to try it out.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>