drm/atomic: Remove pointless private object NULL state check

We will never add private objects with a NULL state into the atomic
state, hence checking for that is pointless.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712155102.26276-2-ville.syrjala@linux.intel.com