drm/bridge: dw-hdmi: fix AVI frame colorimetry

CTA-861-F explicitly states that for RGB colorspace colorimetry should
be set to "none". Fix that.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: def23aa7

 ("drm: bridge: dw-hdmi: Switch to V4L bus format and encodings")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304232512.51616-2-jernej.skrabec@siol.net