drm/omap: convert dss_mgr_unregister_framedone_handler to accept omap_channel

We are removing uses of 'struct omap_overlay_manager'. This patch
changes dss_mgr_unregister_framedone_handler() to accept 'enum
omap_channel' instead of 'struct omap_overlay_manager'.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>