drm: bridge: dw-hdmi: Pass private data pointer to .mode_valid()

Platform glue drivers for dw_hdmi may need to access device-specific
data from their .mode_valid() implementation. They currently have no
clean way to do so, and one driver hacks around it by accessing the
dev_private data of the drm_device retrieved from the connector.

Add a priv_data void pointer to the dw_hdmi_plat_data structure, and
pass it to the .mode_valid() function.

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/20200526011505.31884-12-laurent.pinchart+renesas@ideasonboard.com