i965: Make vertex color clamp handling code VS specific.

i965: Make vertex color clamp handling code VS specific.

Vertex color clamping only applies to gl_[Secondary]{Front,Back}Color,
which are compatibility-only built-in varyings.  We only support GS in
core profile, so they can't exist in geometry shaders.

We can drop several dirty bits from the GS program key - they're
unnecessary for a core profile implementation.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>