OMAPDSS: APPLY: add op->enabling

When we are enabling an overlay, there's a point in time when the
overlay is not actually enabled yet (nor do we want it to be enabled),
but we do want to check if the configuration for the overlay is valid,
and to calculate correct fifo thresholds for the soon-to-be-enabled
overlay.

Current code handled this in a hacky way, setting op->enabled to true
temporarily when calling functions that need to consider the state when
the overlay is enabled.

This patch makes this a bit cleaner, adding "enabling" field, which is
set when the overlay is not yet enabled but should be considered in the
checks and calculations.

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