drm/i915: Clean up SSKPD/MLTR defines

Give names to the SSKPD/MLTR fields, and use the
REG_GENMASK* and REG_FIELD_GET*.

Also drop the bogus non-mirrored SSKP register define.

v2: Rebase due to intel_mchbar_regs.h
    Leave gen6_check_mch_setup() in place for the moment

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220216232806.6194-3-ville.syrjala@linux.intel.com