OMAPDSS: DISPC: check if scaling setup failed

The DISPC's scaling code seems to presume that decimation always
succeeds, and so we always do find a suitable downscaling setup.
However, this is not the case, and the algorithm can fail.

When that happens, the code just proceeds with wrong results, causing
issues later.

Add the necessary checks to bail out if the scaling algorithm failed.

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