drm/i915: Move swizzle_bit under i915_ggtt

The HW performs swizzling as part of its fence tiling inside the Global
GTT. We already do the probing of the HW settings from the GGTT setup,
complete the picture by storing the information as part of the GGTT. The
primary benefit is the consistency of our probe routines do not break
the i915_ggtt 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/20191016143234.4075-2-chris@chris-wilson.co.uk