iris: Add aux offset into hiz_address

iris: Add aux offset into hiz_address

This is not currently required because the hiz buffer is in a separate
buffer, and therefore the offset is 0. If we combine the aux buffer
with the main surface buffer, then the hiz offset may become non-zero.

Suggested-by: Nanley Chery <nanley.g.chery@intel.com>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>