This was hidden originally to workaround a bug in the RK3399 display
driver. The patch resolving this issue has been merged in the upstream
kernel, and in fact...

1. The issue was visible on 21.0 even with this workaround under certain
   configurations (sway with an external monitor).
2. Even on buggy kernels, due to other platform details this is an
   obscure bug to hit (not aware of any ways to trigger it OOTB with
   current userspaces other than sway with an external monitor)

So why bother? Let's just delete the hack and let AFBC be used freely.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Cc: mesa-stable
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <mesa/mesa!10148>