radv: add GFX9 cache flushing support.

radv: add GFX9 cache flushing support.

GFX9 needs to write event EOP to a fence buffer, allocate some
space for this, and just write an ever increasing number to it,
this isn't exactly what radeonsi does, but it seems to work.

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