drm/i915/mst: use the MST mode detected previously

drm/i915/mst: use the MST mode detected previously

Drop the duplicate read of DP_MSTM_CAP DPCD register, and the duplicate
logic for choosing MST mode, and store the chosen mode in struct
intel_dp. Rename intel_dp_configure_mst() to intel_dp_mst_configure()
while at it.

v2: Rebase on drm_dp_mst_detect() returning the mode, not bool

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/93a48df9a77e1138bb28e645fae3f9c79b094cc7.1710839496.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>