drm: rockchip: export functions needed by rockchip dw_hdmi bridge driver

To build the rockchip dw_hdmi driver as a module, the
rockchip_drm_encoder_get_mux_id and rockchip_drm_crtc_mode_config
functions need to be exported.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>