drm/omap: convert dss_mgr_start_update to accept omap_channel

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

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