drm/i915: Don't oops during modeset shutdown after lpe audio deinit

drm/i915: Don't oops during modeset shutdown after lpe audio deinit

BugLink: https://bugs.launchpad.net/bugs/1836802
commit 6a8915d0f8cf323e1beb792a33095cf652db4056 upstream.

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.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit f45a7977d1140c11f334e01a9f77177ed68e3bfa)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>