DSS2: Added support for querying color keying.

V4L2 Framework has a ioctl for getting/setting color keying.
So added the function manager pointers for doing same.

Modifed the color keying sysfs entries to use manager
function pointer. Earlier they were calling direcly
dispc function to set/enable color keying.

Some of color-keying function pointers in the overlay_manager
structure re-named to be more specific.

Signed-off-by: Hardik Shah <hardik.shah@ti.com>