examples/quota_watermark: use ring space for watermarks

Now that the enqueue function returns the amount of space in the ring,
we can use that to replace the old watermark functionality. Update the
example app to do so, and re-enable it in the examples Makefile.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>