i965/vec4: Make a function for setting up vec4 program key clip info.

i965/vec4: Make a function for setting up vec4 program key clip info.

This functionality will need to be reused by geometry shaders.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>