This will come useful in the following patch.

v2:
 * Handle mock ggtt.
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-21-tvrtko.ursulin@linux.intel.com