drm/i915/overlay: Fix oops on unload

drm/i915/overlay: Fix oops on unload

Apparently I failed to test the unload case properly and
thus didn't notice that the i915_overlay_is_active() needs
i915->overlay after fetch_and_zero() already cleared it.
Stop using fetch_and_zero() and only clear the pointer at
the end to avoid the oops.

Fixes: 38d9a352c45e ("drm/i915/overlay: Extract i915_overlay_is_active()")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260303101417.14409-1-ville.syrjala@linux.intel.com
Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>