i965/nir: Pull GLSL uniform handling into a common function

i965/nir: Pull GLSL uniform handling into a common function

The way we deal with GLSL uniforms and builtins is basically the same in
both the vec4 and the fs backend.  This commit takes the best parts of both
implementations and pulls the common code into a shared helper function.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>