Add "updating" flag to mgr_priv_data, which is used to track whether the
manager is currently actively working, i.e. updating the screen.

For auto-update displays updating is always true when the display is
enabled, but for manual update displays the flag is true only during
frame transfers.

The flag is used to manage registers properly, for example when updating
is false and a register for a manager is written, there's no need to set
shadow_dirty flag nor set the GO bit, as the settings will be taken into
use by the HW automatically when the output is enabled.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>