drm/omap: fix plane's channel selection

omap_plane_pre_apply() sets the plane's output channel too late, only
after the plane has already been otherwise configured and enabled. This
causes problems, as at the configuration stage we need to make decisions
based on the output channel.

This may lead to bad plane settings or failing to setup the plane.

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