lima/ppir: use ra_get_best_spill_node to select spill node

lima/ppir: use ra_get_best_spill_node to select spill node

ra_get_best_spill_node is what other users of the mesa register
allocator use.
Switching to it now also fixes an infinite loop issue with ppir regalloc
with the ppir control flow patchset, and also provides a small gain over
the previous herusitic on number of spilled nodes testing with
shader-db.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>