vk: Fall back to previous gens in entry point resolver

vk: Fall back to previous gens in entry point resolver

We used to always just do a one-level fallback from genX_* to anv_*
entry points. That worked for gen7 and gen8 where all entry points were
either different or could be made anv_* entry points (eg
anv_CreateDynamicViewportState). We're about to add gen9 and now need to
be able to fall back to gen8 entry points for most things.

Signed-off-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>