vc4: Switch to using Mesa's register allocator.

vc4: Switch to using Mesa's register allocator.

This will let me more reliably allocate a-file registers, which are going
to be even more in demand when I start using a-file unpacks.

Also fixes a bug where the reservation of payload registers (FRAG_Z/W) was
off by one but just caused failure to register allocate at all if the
off-by-one was fixed.