intel: aubinator: fix read the context/ring

Up to now we've been lucky that the buffer returned was always exactly
at the address we requested.

Fixes: 144b40db ("intel: aubinator: drop the 1Tb GTT mapping")
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
(cherry picked from commit 35955afa)