drm/i915/dp_mst: Enable FEC early once it's known DSC is needed

drm/i915/dp_mst: Enable FEC early once it's known DSC is needed

Enable FEC in crtc_state, as soon as it's known it will be needed by
DSC. This fixes the calculation of BW allocation overhead, in case DSC
is enabled by falling back to it during the encoder compute config
phase (vs. enabling FEC due to DSC being enabled on other streams).

v2:
- Enable FEC only in intel_dp_mst_find_vcpi_slots_for_bpp(), since
  only by that will crtc_state->port_clock be set, which in turn is
  needed by intel_dp_is_uhbr().

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-11-imre.deak@intel.com