i965/vs: Set brw_vs_prog_key::clamp_vertex_color to 0 when irrelevant.

i965/vs: Set brw_vs_prog_key::clamp_vertex_color to 0 when irrelevant.

Vertex color clamping is only relevant if the shader writes to
the built-in gl_[Secondary]{Front,Back}Color varyings.  Otherwise,
brw_vs_prog_key::clamp_vertex_color is never used, so we can simply
leave it set to 0.

This enables us to correctly predict the clamp_vertex_color key value
in the precompile for shaders which don't use those varyings.

Eliminates virtually all VS recompiles in Serious Sam 3's intro.

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