drm/i915/rkl: Handle comp master/slave relationships for PHYs

Certain combo PHYs act as a compensation master to other PHYs and need
to be initialized with a special irefgen bit in the PORT_COMP_DW8
register.  Previously PHY A was the only compensation master (for PHYs
B & C), but RKL adds a fourth PHY which is slaved to PHY C instead.

Bspec: 49291
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Aditya Swarup <aditya.swarup@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603211529.3005059-12-matthew.d.roper@intel.com