drm/bridge: dw-hdmi: do not force "none" scan mode

Setting scan mode to "none" confuses some TVs like LG B8, which randomly
change overscan percentage over time. Digital outputs like HDMI and DVI,
handled by this controller, don't really need overscan, so we can always
set scan mode to underscan. Actually, this is exactly what
drm_hdmi_avi_infoframe_from_display_mode() already does, so we can just
remove offending line.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
[updated commit message]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304232512.51616-3-jernej.skrabec@siol.net