drm/atomic: Make private objs proper objects

Make the atomic private object stuff less special by introducing proper
base classes for the object and its state. Drivers can embed these in
their own appropriate objects, after which these things will work
exactly like the plane/crtc/connector states during atomic operations.

v2: Reorder to not depend on drm_dynarray (Daniel)

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712155102.26276-3-ville.syrjala@linux.intel.com