drm/i915/mst: abstract choosing the MST mode to use

Clarify the conditions for choosing the MST mode to use by adding a new
function intel_dp_mst_mode_choose(). This also prepares for being able
to extend the MST modes to single-stream sideband messaging.

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/f626144f10b03d4609ff38a29bac013ecf3aca4e.1710839496.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>