OMAPDSS: remove enum omap_dss_overlay_managers

We have two almost the same enums: omap_channel and
omap_dss_overlay_managers. omap_channel is used almost everywhere, and
omap_channel assigns explicit values to the enum values which are needed
for proper operation.

omap_dss_overlay_managers is only used in one place, so it's easy to
remove it, which is what this patch does.

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