Add zorder support on OMAP4, this feature allows deciding the visibility order
of the overlays based on the zorder value provided as an overlay info parameter
or a sysfs attribute of the overlay object.

Use the overlay cap OMAP_DSS_OVL_CAP_ZORDER to determine whether zorder is
supported for the overlay or not. Use dss feature FEAT_ALPHA_FREE_ZORDER
if the caps are not available.

Ensure that all overlays that are enabled and connected to the same manager
have different zorders. Swapping zorders of 2 enabled overlays currently
requires disabling one of the overlays.
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>