drm/i915: Add missing invalidate to g4x wm readout

Let's not forget to mark the unused watermark levels as invalid
after the readout. The vlv/chv codepath has this but the g4x
didn't for some reason.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220622155452.32587-6-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>