OMAPDSS: APPLY: move channel-field to extra_info set

Setting overlay's output channel is currently handled at the same time
as other overlay attributes. This is not right, as the normal attributes
should only affect one overlay and manager, but changing the channel
affects two managers.

This patch moves the channel field into the "extra_info" set, handled
together with enabled-status.

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