FB: OMAP: Blizzard: Properly disable zoom when necessary

The problem is related to the following notice in the documentation:
"Once a destructive window with up-scaling is created, it can only be
disabled by creating a background window.". With the current omapfb
driver code, if you try to do a partial screen update (show menu or
dialog) after a scaled screen update (video overlay), scaling setting
are not reset properly and you see this "magnifying glass" effect if a
part of new update overlaps previously scaled area.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@nokia.com>
Signed-off-by: Daniel Stone <daniel.stone@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>