drm/i915/selftests: Set vm->gt backpointer for mock_ppgtt

Add the backpointer to ppgtt and i915->gt so that we can traverse across
the device hierarchy.

Reported-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191022095851.23442-1-chris@chris-wilson.co.uk