anv/entrypoints: Better handle promoted extensions

anv/entrypoints: Better handle promoted extensions

In the case of promoted extensions we can end up with an entrypoint that
we support being an alias of an entrypoint we do not support.  For
instance, if an extension gets promoted from EXT to KHR, the EXT entry-
points may be aliases of the KHR ones.  We want to leave everything as
EXT until we get around to advertising the KHR so that we don't break
things when we update the XML and headers.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>