OMAPDSS: APPLY: rename overlay_cache_data

overlay_cache_data is not a suitable name for the struct. It is more of
a private data for the overlay.

Rename the struct to ovl_priv_data, and add a function,
get_ovl_priv(ovl), to get a pointer to the data.

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