drm/bridge: dw-hdmi: disable SCDC configuration for invalid setups

This patch is an attempt to limit HDMI 2.0 SCDC setup when :
- the SoC embeds an HDMI 1.4 only controller
- the EDID supports SCDC but not scrambling
- the EDID supports SCDC scrambling but not for low TMDS bit rates,
  while only supporting low TMDS bit rates

This to avoid communicating with the SCDC DDC slave uncessary, and
setting the DW-HDMI TMDS Scrambler setup when not supported by the
underlying hardware.

Reported-by: Rob Herring <robh@kernel.org>
Fixes: 264fce6c ("drm/bridge: dw-hdmi: Add SCDC and TMDS Scrambling support")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190315095414.28520-1-narmstrong@baylibre.com