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

 drivers/gpu/drm/rockchip/rockchip_rgb.c:71:22: warning: no previous prototype for ‘rockchip_rgb_init’ [-Wmissing-prototypes]
 drivers/gpu/drm/rockchip/rockchip_rgb.c:159:6: warning: no previous prototype for ‘rockchip_rgb_fini’ [-Wmissing-prototypes]

Cc: Sandy Huang <hjc@rock-chips.com>
Cc: "Heiko Stübner" <heiko@sntech.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.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/20201116174112.1833368-26-lee.jones@linaro.org