anv: Refactor descriptor pushing a bit
Pull the common code out of the two entrypoints into the helper which
fetches the push descriptor set for us. Now that it does more than just
get a thing, call it anv_cmd_buffer_push_descriptor_set.
Cc: "19.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by:
Lionel Landwerlin <lionel.g.landwerlin@intel.com>