i965/nir: Pull common ARB program uniform handling into a common function

i965/nir: Pull common ARB program uniform handling into a common function

The way we deal with ARB program uniforms 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>