drm/i915: Use REG_BIT() & co for the pre-ilk pfit registers

Modernize the gmch pfit register definitions using REG_BIT/etc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230418175528.13117-6-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>