drm/i915: re-enable -Wunused-but-set-variable

W=1 enables -Wunused-but-set-variable. We disabled it locally in i915
Makefile as we were hitting a bunch of warnings. See commit 6a05d290
("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.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/28ca3f95fe77ceb8aa35b87fca73f7afbc89859a.1685119007.git.jani.nikula@intel.com