OMAPDSS: DISPC: zorder support for DSS overlays

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>