drm/omap: convert dss_mgr_ops to use omap_channel

We are removing the use of 'struct omap_overlay_manager' from omapdrm.
This patch changes the function pointers in 'dss_mgr_ops' to get 'enum
omap_channel' parameter instead of 'struct omap_overlay_manager'.

The change is very straightforward, as we still use 'struct
omap_overlay_manager' inside the function implementations where needed.

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