drm/shmob: Nuke preclose hook

Again since the drm core takes care of event unlinking/disarming this
is now just needless code.

v2: Fixup misplaced hunk.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-11-git-send-email-daniel.vetter@ffwll.ch