drm: bridge: dw-hdmi: remove unused hdmi_bus_fmt_is_yuv420

Remove usused yet hdmi_bus_fmt_is_yuv420 function.

Fixes: def23aa7 ("drm: bridge: dw-hdmi: Switch to V4L bus format and encodings")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1491377579-9353-1-git-send-email-narmstrong@baylibre.com