We deinit the lpe audio device before we call
drm_atomic_helper_shutdown(), which means the platform device
may already be gone when it comes time to shut down the crtc.
As we don't know when the last reference to the platform
device gets dropped by the audio driver we can't assume that
the device and its data are still around when turning off the
crtc. Mark the platform device as gone as soon as we do the
audio deinit.

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181105194604.6994-1-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>