drm/i915/lpe: Mark LPE audio runtime pm as "no callbacks"

drm/i915/lpe: Mark LPE audio runtime pm as "no callbacks"

The LPE audio is a child device of i915, it is powered up and down
alongside the igfx and presents no independent runtime interface. This
aptly fulfils the description of a "No-Callback" Device, so mark it
thus.

Fixes: d4becb50249c ("drm/i915: Fix runtime PM for LPE audio")
Testcase: igt/pm_rpm/basic-pci-d3-state
Testcase: igt/pm_rpm/basic-rte
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180802140416.6062-1-chris@chris-wilson.co.uk