ac: Move has_local_buffers disable to radeonsi.

In radv we had a separate flag to actually use it + an env option
to experimentally use it.

The common code setting has_local_buffers to false of course broke
that experimental option.

Also the "enable on APU" did not make sense for RADV as it is still
disabled by default.

Fixes: b21a4efb "radv/winsys: allow local BOs on APUs"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit af9534b9)