drm/omap: Nuke close hooks

Again since the core takes care of this we can remove them. While at
it also remove the postclose hook, it's empty.

v2: Laurent pointed me at even more code to delete.

v3: Remove unused flags (Tomi).

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453756616-28942-9-git-send-email-daniel.vetter@ffwll.ch