drm/i915/dp: Ensure the FEC state stays disabled for UHBR links

drm/i915/dp: Ensure the FEC state stays disabled for UHBR links

Atm, in the DP SST case the FEC state is computed before
intel_crtc_state::port_clock is initialized, hence intel_dp_is_uhbr()
will always return false and the FEC state will be always computed
assuming a non-UHBR link.

This happens to work, since the FEC state is recomputed later in
intel_dp_mtp_tu_compute_config(), where port_clock will be set already,
so intel_crtc_state::fec_enable will be reset as expected for UHBR. This
also depends on link rates being tried in an increasing order (i.e. from
non-UHBR -> UHBR link rates) in dsc_compute_link_config(), thus
intel_crtc_state::fec_enable being set for the non-UHBR rates and
getting reset for the first UHBR rate as expected.

A follow-up change will reuse intel_dp_fec_compute_config() for the DP
MST state computation, prepare for that here, making sure that the
function determines the correct intel_crtc_state::fec_enable=false state
for UHBR link rates based on the above.

The DP SST and MST state computation should be further unified to avoid
computing/setting the intel_crtc_state::fec_enable state multiple times,
but that's left for a follow-up change. For now add only code comments
about this.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/20251015161934.262108-3-imre.deak@intel.com