drm/i915/dp: Wait for FEC detected status in the sink

As required by the DP standard wait for the sink to detect the FEC
decode enabling symbol sent by the source.

There is a difference between SST and MST when the source enables
the FEC encoding: on SST this happens only after enabling the
transcoder, whereas on MST it happens already after enabling the
transcoder function (before enabling the transcoder). Wait for the
detected status at the earliest spot accordingly.

v2:
- Wait for the FEC detected status on SST after the transcoder is
  enabled.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-20-imre.deak@intel.com