st/mesa: add a second pipeline for compute

st/mesa: add a second pipeline for compute

Compute needs a new and different validation path.

Changes from v2:
 - make use of unreachable() instead of assert() when the pipeline is
   invalid
 - move the st_pipeline enumeration to st_context.h instead of st_api.h

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>