radv/ac: drop setting xnack

radv/ac: drop setting xnack

Since radv uses compute rings and we can't know when we are setting
up the shaders what ring they are to be used on, we should just use
the default xnack setting. This may be suboptimal in some places,
but if we hit a problem, we likely should try and address this
between llvm and mesa.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>