Helper routine to read out maximum supported pixel rate
for DisplayPort legay VGA converter or TMDS clock rate
for other digital legacy converters. The helper returns
clock rate in kHz.

v2: Return early if detailed port cap info is not available.
    Replace if-else ladder with switch-case (Ville)
Reviewed-by: NJim Bride <jim.bride@linux.intel.com>
Signed-off-by: NMika Kahola <mika.kahola@intel.com>
Acked-by: NDave Airlie <airlied@gmail.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-4-git-send-email-mika.kahola@intel.com