drm/atomic-helper: Fix sparse warnings

drm/drm_atomic_helper.c:1696:6: warning: symbol 'plane_crtc_active' was not declared. Should it be static?

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1474292035-15695-2-git-send-email-ville.syrjala@linux.intel.com