i965: Define the setup_vector_uniform_values() backend_visitor interface.

i965: Define the setup_vector_uniform_values() backend_visitor interface.

This cleans up the VEC4 implementation of setup_uniform_values()
somewhat and will avoid duplication of the image uniform upload code
by having a common interface to upload a vector of uniforms on either
back-end.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>