drm/meson: fix TMDS clock filtering for DMT monitors

DMT monitors does not necessarely report a maximum TMDS clock
in a VSDB EDID extension.

In this case, all modes are wrongly rejected, including
the DRM fallback EDID.

This patch only rejects modes whith clock > max_tmds_clock if
the max_tmds_clock is specified. This will only reject
4:2:0 HDMI2.0 modes, who reports a clock > max_tmds_clock.

Reported-by: Maxime Jourdan <mjourdan@baylibre.com>
Fixes: d7d8fb70

 ("drm/meson: add HDMI div40 TMDS mode")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Maxime Jourdan <mjourdan@baylibre.com>
Reviewed-by: Maxime Jourdan <mjourdan@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190320081110.1718-1-narmstrong@baylibre.com