drm/i915/gtt: Onionify error handling for gen6_ppgtt_create

Pull the empty stubs together into the top level gen6_ppgtt_create, and
tear each one down on error in proper onion order (rather than use
Joonas' pet hate of calling the cleanup function in indeterminable
state).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180612081815.3585-2-chris@chris-wilson.co.uk