drm/atmel: Nuke preclose

The only thing this did was cancle pending flip events, and the core
takes care of that now.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-5-git-send-email-daniel.vetter@ffwll.ch