LPT/WPT only have PCH transcoder A. Make sure we poke at its
chicken register instead of some non-existent register when
FDI is being driven by pipe B or C.

Cc: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128182358.14477-1-ville.syrjala@linux.intel.comReviewed-by: Uma Shankar <uma.shankar@intel.com>