OMAP4: OMAPFB: Set overlay zorder to 0 if omapfb API used

Since there's no ioctl to set zorder when modifying overlays
using SETUP_PLANE, it's more useful to userland code if
overlay is 0, so they don't have to use sysfs and first
figure out which overlay is being used by omapfb.

Change-Id: I1c2eeecb0b950439e40eec895d6b89c73df2fd7e
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>