drm/i915: Report aliasing ppgtt size as ggtt size

The aliasing-ppgtt is constrained to be the same size as the Global GTT
since it aliases the same address space. Simplifying gtt size reporting
in this case.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190902040303.14195-2-chris@chris-wilson.co.uk