intel/fs: use pull constant locations to check for first compile of a shader

intel/fs: use pull constant locations to check for first compile of a shader

Before, we bailing in assign_constant_locations based on the minimum
dispatch size.  The more direct thing to do is simply to check for
whether or not we have constant locations and bail if we do.  For
nir_setup_uniforms, it's completely safe to do it multiple times because
we just copy a value from the NIR shader.

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