drm/i915/selftests: Use to_gt() helper for GGTT accesses

GGTT is currently available both through i915->ggtt and gt->ggtt, and we
eventually want to get rid of the i915->ggtt one.
Use to_gt() for all i915->ggtt accesses to help with the future
refactoring.

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211219212500.61432-6-andi.shyti@linux.intel.com