Clear the notify function pointer in the platform data before we tear
down the driver. Otherwise i915 would end up calling a stale function
pointer and possibly explode.

Cc: stable@vger.kernel.org
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170427160231.13337-3-ville.syrjala@linux.intel.comReviewed-by: Takashi Iwai <tiwai@suse.de>