drm/i915: Remove unused i915->ggtt

The reference to the GGTT from the private date is not used
anymore. Remove it.

The ggtt in the root gt will now be dynamically allocated and the
deallocation handled by the drmm_* managed allocation.

Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@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/20211219212500.61432-7-andi.shyti@linux.intel.com