drm: bridge: dw-hdmi: constify copied structure

The dw_hdmi_hw structure is only copied into another structure,
so make it const.

The opportunity for this change was found using Coccinelle.

Fixes: 7ed6c665 ("drm: bridge/dw_hdmi-ahb-audio: add audio driver")
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1577864614-5543-16-git-send-email-Julia.Lawall@inria.fr