drm/i915/dg2: Add MPLLB programming for SNPS PHY

DG2's SNPS PHYs incorporate a dedicated port PLL called MPLLB which
takes the place of the shared DPLLs we've used on past platforms.  Let's
add the MPLLB programming sequences; they'll be plugged into the rest of
the code in future patches.

Bspec: 54032
Bspec: 53881
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Nidhi Gupta <nidhi1.gupta@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723174239.1551352-24-matthew.d.roper@intel.com