Fixes the following link error when CONFIG_DRM_ANALOGIX_DP is not defined:

ERROR: "analogix_dp_start_crc" [drivers/gpu/drm/rockchip/rockchipdrm.ko] undefined!
ERROR: "analogix_dp_stop_crc" [drivers/gpu/drm/rockchip/rockchipdrm.ko] undefined!

Fixes: 3190e58d ("drm/rockchip: Implement CRC debugfs API")
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Mark Yao <mark.yao@rock-chips.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>