OMAP: DSS2: Don't allow moving managers away from enabled displays

If a manager is moved while attached to an enabled display, the DSS
system will be left in an inconsistent state. This will eventually cause
a kernel oops when the enabled display is disabled.

Fix this by not allowing the user to move a manager away from an enabled
display.

Signed-off-by: Daniel Morsing <daniel.morsing@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>