OMAP: DSS2: check for manager when enabling display

OMAP: DSS2: check for manager when enabling display

None of the DSS interface drivers check if an overlay manager is
connected to the display when the display is being enabled. This leads
to null pointer crash if the display has no manager.

This patch checks for the manager and returns an error if it is null.

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