drm/i915/hdmi: Go for scrambling only if platform supports TMDS clock > 340MHz

There are cases, where devices have an HDMI1.4 retimer, and TMDS clock rate
is capped to 340MHz via VBT. In such cases scrambling might be supported
by the platform and an HDMI2.0 sink for lower TMDS rates, but not
supported by the retimer, causing blankouts.

So avoid enabling scrambling, if the TMDS clock is capped to <= 340MHz.

v2: Added comment, documenting the rationale to check for TMDS clock,
before going for scrambling. (Arun)

v3: Fixed the function name to check if source supports scrambling. (Jani)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221222040851.3029514-1-ankit.k.nautiyal@intel.com