drm/i915: add a note about fec_enable with 128b/132b

Add a note that fec_enable actually means FEC is to be enabled
explicitly. 128b/132b always has FEC enabled, the driver doesn't need to
enable it separately, and fec_enable will be false.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230920112901.3315876-1-jani.nikula@intel.com