OMAP4: OMAPFB: Always set a x_decim and y_decim in overlays

When using the framebuffer API to setup overlay, there's
currently no way to pass a x_decim and y_decim value in
so set some safe non-zero defaults.  Otherwise, when
the overlay is enabled, zero values will cause divide
by zero error in dispc.

Change-Id: Id2cb4bc4cb6be7b57e3ab5fc28fffc57163faecb
Signed-off-by: Mike J. Chen <mjchen@google.com>
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>