Helper routine to read out maximum supported bits per
component for DisplayPort legay converters.

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/1473419458-17080-5-git-send-email-mika.kahola@intel.com