drm/armada/armada_overlay: Staticify local function 'armada_overlay_duplicate_state'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/armada/armada_overlay.c:329:1: warning: no previous prototype for ‘armada_overlay_duplicate_state’ [-Wmissing-prototypes]

Cc: Russell King <linux@armlinux.org.uk>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201116174112.1833368-3-lee.jones@linaro.org

drivers/gpu/drm/armada/armada_overlay.c

@@ -325,7 +325,7 @@ static void armada_overlay_reset(struct drm_plane *plane)
 	}
 }
 
-struct drm_plane_state *
+static struct drm_plane_state *
 armada_overlay_duplicate_state(struct drm_plane *plane)
 {
 	struct armada_overlay_state *state;