st/glsl_to_nir: simplify st_nir_assign_var_locations() and fix for fs outputs
We only need to check for previously processed location on user
defined varyings as they are the only ones that support component
packing. Therefore a single instance of processed_locs can be
shared by regular varyings and patches.
For simplicity we make processed_locs an array in order to handle
dual source bleanding.
Fixes the follow piglit test on radeonsi:
tests/spec/arb_enhanced_layouts/execution/component-layout/fs-output.shader_test
Reviewed-by: Dave Airlie <airlied@redhat.com>