Fix divisions by zero on boot.

Change I976ffae3a56d411b00026f86b16efa9743a23293 introduced a problem
where depending on the rotation settings the WM override display
configuration would be filled with invalid values.
Fix that by correctly checking that the display is ready prior to
setting the override configuration.

Change-Id: Ia3fd7603d9c1212cd32536803b4c12390a798606