drm: bridge: dw-hdmi: Constify mode argument to internal functions

Several internal functions take a drm_display_mode argument to configure
the HDMI encoder or the HDMI PHY. They must not modify the mode, make
the pointer const to enforce 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/20200526011505.31884-18-laurent.pinchart+renesas@ideasonboard.com