drm/i915/selftests: Remove hardcoded value with a macro

Use MI_USE_GGTT instead of hardcoded value.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221213120010.5857-1-nirmoy.das@intel.com