OMAPDSS: APPLY: rename manager_cache_data

manager_cache_data is not a suitable name for the struct. It is more of
a private data for the manager.

Rename the struct to mgr_priv_data, and add a function,
get_mgr_priv(mgr), to get a pointer to the data.

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