drm/i915: Store ggtt pointer in intel_gt

This will become useful in the following patch.

v2:
 * Assign the pointer through a helper on the top level to work around
   the layering violation. (Chris)

v3:
 * Handle selftests.

v4:
 * Move call to intel_gt_init_hw into mock_init_ggtt. (Chris)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-28-tvrtko.ursulin@linux.intel.com