anv: Use an actual binding for gl_NumWorkgroups

anv: Use an actual binding for gl_NumWorkgroups

This commit moves our handling of gl_NumWorkgroups over to work like our
handling of other special bindings in the Vulkan driver.  We give it a
magic descriptor set number and teach emit_binding_tables to handle it.
This is better than the bias mechanism we were using because it allows
us to do proper accounting through the bind map mechanism.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>