OMAPDSS: add omapdss_compat_init()

Add two new exported functions, omapdss_compat_init and
omapdss_compat_uninit, which are to be used by omapfb, omap_vout to
enable compatibility mode for omapdss. The functions are called by
omapdss internally for now, and moved to other drivers later.

The compatibility mode is implemented fully in the following patches.
For now, enabling compat mode only sets up the private data in apply.c.

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