radv: Replace supports_spill with explict_scratch_args

radv: Replace supports_spill with explict_scratch_args

The former was always true and hence dead code. We will want to
explicitly declare the ring offset register with ACO, but we also want
to declare the scratch offset too, and we can't try to disable it since
ACO also supports spilling and the determination of whether spilling has
to happen occurs well after setting up registers. So replace
supports_spill with something that will actually be used for ACO.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>