HyperZ is an undocumented feature (outside of ATI) of the radeon
chips, this is a reverse engineered implementation.

From: Roland Scheidegger <rscheidegger_lists@hispeed.ch> and
Stephane Marchesin <marchesin@icps.u-strasbg.fr>
Signed-off-by: Dave Airlie <airlied@linux.ie>