drm/i915/dp: Fix determining SST/MST mode during MTP TU state computation

drm/i915/dp: Fix determining SST/MST mode during MTP TU state computation

Determining the SST/MST mode during state computation must be done based
on the output type stored in the CRTC state, which in turn is set once
based on the modeset connector's SST vs. MST type and will not change as
long as the connector is using the CRTC. OTOH the MST mode indicated by
the given connector's intel_dp::is_mst flag can change independently of
the above output type, based on what sink is at any moment plugged to
the connector.

Fix the state computation accordingly.

Cc: Jani Nikula <jani.nikula@intel.com>
Fixes: f6971d7427c2 ("drm/i915/mst: adapt intel_dp_mtp_tu_compute_config() for 128b/132b SST")
Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/4607
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250507151953.251846-1-imre.deak@intel.com
(cherry picked from commit 0f45696ddb2b901fbf15cb8d2e89767be481d59f)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>