i965: Enable scalar GS by default.

i965: Enable scalar GS by default.

I'd originally left this off because Orbital Explorer was hanging the
GPU, but it seems to be working these days.  There have been a bunch
of changes since then, so we probably fixed something.

On my Broadwell laptop, both Synmark/GSCloth and Orbital Explorer seem
to run at approximately the same framerate in either mode.  This is
despite large reductions in instruction count for Synmark, and large
increases for Orbital Explorer.  It apparently just doesn't matter.

Switching to scalar mode will gain us fp64 support in the next release,
as vec4-mode support isn't yet ready.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>