radv: Add userspace fence buffer per context.

radv: Add userspace fence buffer per context.

Having it in the winsys didn't work when multiple devices use
the same winsys, as we then have multiple contexts per queue,
and each context counts separately.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Fixes: 7b9963a28f4 "radv: Enable userspace fence checking."