drm/i915/dp: Simplify forcing a link retraining

drm/i915/dp: Simplify forcing a link retraining

Since both the DP SST and MST HPD IRQ handlers call
intel_dp_handle_link_service_irq() with LINK_STATUS_CHANGED set in
irq_mask if intel_dp->link.force_retrain is set, checking for the former
flag is sufficient to determine if the link status needs to be checked
(which includes retraining the link if this is forced); remove checking
for the latter flag.

Since LINK_STATUS_CHANGED is currently set unconditionally for DP SST,
extend the related comment to note that it must be set if
intel_dp->link.force_retrain is set (in case setting LINK_STATUS_CHANGED
becomes conditional on DPCD_REV).

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260311153152.133744-2-imre.deak@intel.com