drm/i915/snps: use div32 version of MPLLB word clock for UHBR

The mode set sequence for 128b/132b requires setting the div32 version
of MPLLB clock.

Bspec: 53880, 54128
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211202144456.2541305-1-jani.nikula@intel.com