drm/i915: Remove nop bigjoiner state copy

We just copied over the whole master crtc state, including
cpu_transcoder+has_audio. No need to copy those again.

Also get rid of the unhelpful comment.

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-4-ville.syrjala@linux.intel.com