W=1 enables -Wunused-but-set-variable. We disabled it locally in i915
Makefile as we were hitting a bunch of warnings. See commit 6a05d2900464
("drm/i915: Disable unused-but-set compiler warning").
With the issues fixed or annotated with __maybe_unused, re-enable the
warning not only in W=1 but also locally as part of i915 build.