drm/i915: Store i915_ggtt as the backpointer on fence registers

Now that i915_ggtt knows everything about its own paths to perform mmio,
we can use that as our primary backpointer for individual fence
registers. This reduces the amount of pointer dancing we have to perform
on the common paths, but more importantly finishes our fence register
encapsulation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016143234.4075-1-chris@chris-wilson.co.uk