drm/i915: Disable unused-but-set compiler warning

The unused-but-set warning enabled by W=1 catches out a lot of the
atomic helper iterator macros and drown us in their noise (or trip over
Werror and die). Path of least resistance is to ignore the warning.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208161639.27511-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>