OMAPFB: fix overlay disable when freeing resources.

When omapfb is shutting down, it will disable all the overlays. However,
instead of actually disabling all the overlays, it disables only all the
overlays that are currently attached to framebuffers.

On OMAP4+, this leaves the fourth overlay left enabled.

Fix the loop so that we actually go through all the overlays.

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