drm/i915/gtt: Onionify error handling for gen6_ppgtt_create

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 <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Matthew Auld <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]