drm/blend: Fix sparse warnings

drm/drm_blend.c:207:5: warning: symbol 'drm_atomic_normalize_zpos' was not declared. Should it be static?

Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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/1474292035-15695-3-git-send-email-ville.syrjala@linux.intel.com