drm/i915: Program chicken bit during DP MST sequence on TGL+

drm/i915: Program chicken bit during DP MST sequence on TGL+

A new step has been added to the DP modeset sequences for all platforms
with display version 12 and beyond:  if enabling DP MST with FEC, we
need to set a chicken bit before enabling the transcoder.  The chicken
bit should be disabled again before disabling the transcoder (which we
can do unconditionally since it shouldn't be set anyway in non-MST
cases).

Bspec: 49190, 54128, 55424
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723170618.1477415-1-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>