drm/atomic: Set current atomic state in drm_private_state

drm_private_state has a back pointer to the drm_atomic_state,
however that was not initialized in drm_atomic_get_private_obj_state
after duplication, as it is the case for other drm atomic getters

Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1527701452-1934-1-git-send-email-alexandru-cosmin.gheorghe@arm.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>