anv/entrypoints: Rework #if guards

anv/entrypoints: Rework #if guards

This reworks the #if guards a bit.  When Emil originally wrote them, he
just guarded everything.  However, part of what anv_entrypoints_gen.py
generates is a hash table for looking up entrypoints based on their name.
This table *cannot* get out of sync between C and python regardless of
preprocessor flags.  In order to prevent this, this commit makes us use
void pointers in the dispatch table for those entrypoints which aren't
available.  This means that the dispatch table size and entry order is
constant and it should never get out-of-sync with the python.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 8d37556ec9d7fbbffc5497388a52998ae4fe75de)