intel/compiler: Add a flag for pull constant support

intel/compiler: Add a flag for pull constant support

The Vulkan driver does not support pull constants.  It simply limits
things such that we can always push everything.  Previously, we were
determining whether or not to push things based on whether or not the
prog_data::pull_param array is non-null.  This is rather hackish and
about to stop working.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>