drm/omap: Add global state as a private atomic object

Global shared resources (like hw overlays) for omapdrm are implemented
as a part of atomic state using the drm_private_obj infrastructure
available in the atomic core.

omap_global_state is introduced as a drm atomic private object. The two
funcs omap_get_global_state() and omap_get_existing_global_state() are
the two variants that will be used to access omap_global_state.

drm_mode_config_init() needs to be called earlier because it
creates/initializes the private_obj link list maintained by the atomic
framework. The private_obj link list has to exist prior to calling
drm_atomic_private_obj_init(). Similarly the cleanup handler are
reordered appropriately.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117141928.771082-7-narmstrong@baylibre.com