drm/i915: Perform correct cpu_transcoder readout for bigjoiner

Read out cpu_transcoder correctly for the bigjoiner slave pipes.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211022103304.24164-8-ville.syrjala@linux.intel.com