drm/i915: Store intel_gt backpointer in vm

This will come useful in the following patch.

v2:
 * Handle mock ggtt.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-21-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/selftests/mock_gtt.c

@@ -98,6 +98,7 @@ void mock_init_ggtt(struct drm_i915_private *i915, struct i915_ggtt *ggtt)
 {
 	memset(ggtt, 0, sizeof(*ggtt));
 
+	ggtt->vm.gt = &i915->gt;
 	ggtt->vm.i915 = i915;
 	ggtt->vm.is_ggtt = true;