drm/i915: Use the correct PCH transcoder for LPT/WPT in intel_sanitize_frame_start_delay()

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.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>