drm/i915: Make ilk+ pfit regiser unlocked

The ilk+ panel fitter register are sitting nicely on their own
cacheline, so no need for global serialization via uncore.lock.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220224165103.15682-3-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>