drm/i915: Amalgamate GGTT/ppGTT vma debug list walkers

As we can now have multiple VMA inside the global GTT (with partial
mappings, rotations, etc), it is no longer true that there may just be a
single GGTT entry and so we should walk the full vma_list to count up
the actual usage. In addition to unifying the two walkers, switch from
multiplying the object size for each vma to summing the bound vma sizes.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-1-git-send-email-chris@chris-wilson.co.uk