drm/i915/fbc: Polish the skl+ FBC stride override handling

drm/i915/fbc: Polish the skl+ FBC stride override handling

Polish the FBC stride override stuff:
- just call it override_cfb_stride since it'll be used on
  more gens later
- Use REG_BIT() & co. for the registers and give everything
  CHICKEN_ prefix since glk+ will have a different register
  for this
- Use intel_de_rmw() for the RMW

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702204603.596-5-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>