drm: meson: dw-hdmi: Use dw_hdmi context to replace hack

The meson-dw-hdmi driver needs to access its own context from the
.mode_valid() operation. It currently gets it from the dev_private field
of the drm_device retrieved from the connector, which is a hack. Use the
private data passed to the .mode_valid() operation instead.

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/20200526011505.31884-15-laurent.pinchart+renesas@ideasonboard.com