drm/i915: Add distinct stubs for PM hibernation phases

Currently for handling the extra hibernation phases we just call the
equivalent suspend/resume phases. In the next couple of patches, I wish
to specialise the hibernation phases to reduce the amount of work
required for handling GEM objects.

v2: There are more! Don't forget the freeze phases.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: David Weinehall <david.weinehall@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463207195-22076-1-git-send-email-chris@chris-wilson.co.uk