drm/i915/adl_p: enable MSO on pipe B

On ADL-P, it's possible to enable the stream splitter on pipe B in
addition to pipe A.

Bspec: 50174
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210526082903.26395-1-jani.nikula@intel.com