anv: Restrict the number of color regions to those actually written

anv: Restrict the number of color regions to those actually written

The back-end compiler emits the number of color writes specified by
wm_prog_key::nr_color_regions regardless of what nir_store_outputs we
have.  Once we've gone through and figured out which render targets
actually exist and are written by the shader, we should restrict the key
to avoid extra RT write messages.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>