drm/i915/gt: 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.

During the probe of i915 the early intiialization of the gt
(intel_gt_init_hw_early()) is moved prior to any access to the
ggtt. This because it's in that moment we assign the ggtt to the
gt and we want to do that before using it.

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/20211221195946.3180-1-andi.shyti@linux.intel.com