drm: bridge: dw-hdmi: Constify mode argument to dw_hdmi_phy_ops .init()

The PHY .init() must not modify the mode it receives. Make the pointer
const to enfore that.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200526011505.31884-17-laurent.pinchart+renesas@ideasonboard.com