i965/vec4: Extract function to set up vec4 prog key for precompiling.

i965/vec4: Extract function to set up vec4 prog key for precompiling.

This will allow us to re-use it for precompiling geometry shaders.

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