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