Setting overlay's fifo thresholds 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 fifo
thresholds are needed in cases like fifo-merge, where multiple managers
are affected.

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

This also lets us to calculate the fifos only when needed, specifically,
when an overlay or a manager is enabled.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>