radv/winsys: always allow GTT placements on APUs

radv/winsys: always allow GTT placements on APUs

When the VRAM size is small and the preferred heap only VRAM,
the kernel tries to always honor the requested heap and it does
a ton of evictions which is a disaster for performance.

On APUs, VRAM and GTT have similar performance, so allow the
kernel to choose the best placement (GTT or VRAM) itself.

This gives a huge performance boost with Doom Eternal on RAVEN.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5665>