OMAPDSS: DISPC: fix div by zero issue in overlay scaling

omapdrm doesn't always configure the overlays correctly, causing the
overlay setup functions to be called with zero timings. This leads to
division by zero error.

This happens, for example, when a HDMI cable is not connected, but a
user tries to setup a plane with scaling.

Fixing omapdrm is a big job, so for now let's check for the bad timings
in DISPC and return an error.

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