i965/vs: Handle vertex color clamping in emit_urb_slot().

i965/vs: Handle vertex color clamping in emit_urb_slot().

Vertex color clamping only applies to a few specific built-ins: COL0/1
and BFC0/1 (aka gl_[Secondary]{Front,Back}Color).  It seems weird to
handle special cases in a function called emit_generic_urb_slot().

emit_urb_slot() is all about handling special cases, so it makes more
sense to handle this there.

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