drm: Add drm_atomic_get_old/new_private_obj_state

This pair of functions return the old/new private object state for the
given private_obj, or NULL if the private_obj is not part of the global
atomic state.

Reviewed-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>