drm/i915/mst: improve debug logging of DP MST mode detect

drm/i915/mst: improve debug logging of DP MST mode detect

Rename intel_dp_can_mst() to intel_dp_mst_detect(), and move all DP MST
detect debug logging there. Debug log the sink's MST capability,
including single-stream sideband messaging support, and the decision
whether to enable MST mode or not. Do this regardless of whether we're
actually enabling MST or not.

We need to detect MST in intel_dp_detect_dpcd() before the earlier
returns, but try not to change the logic otherwise.

v2:
- Use "MST", "SST w/ sideband messaging", and "SST" for logging (Ville)
- Return MST mode from intel_dp_mst_detect()
- Do MST detect before early returns from intel_dp_detect_dpcd()

Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/db08536daec0a6062539319d71c10ee1277e3876.1710839496.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>