ra: cleanup the public API

ra: cleanup the public API

Previously, there were 3 entrypoints into parts of the actual allocator,
and an API called ra_allocate_no_spills() that called all 3. Nobody
would ever want to call any of the 3 entrypoints by themselves, so
everybody just used ra_allocate_no_spills(). So just make them static
functions, and while we're at it rename ra_allocate_no_spills() to
ra_allocate() since there's no equivalent "with spills," because the
backend is supposed to handle spilling.

Signed-off-by: Connor Abbott <connor.abbott@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>