intel/eu: Rework surface descriptor helpers

intel/eu: Rework surface descriptor helpers

This commit pulls the surface descriptor helpers out into brw_eu.h and
makes them no longer depend on the codegen infrastructure.  This should
allow us to use them directly from the IR code instead of the generator.
This change is unfortunately less mechanical than perhaps one would like
but it should be fairly straightforward.

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