Add a helper function dss_apply_ovl_enable(), which is similar to the
main apply() function: dss_apply_ovl_enable() applies the given overlay
enable-status to ovl_priv_data, and sets the dirty flag.

The difference between the helper function and the previous direct use
of the fields is that the helper function will not do anything if the
enable state is already the same as given in the parameter.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>