drm/i915/gt: Suppress warnings for unused debugging locals

With debugging turned off, we have to tell the compiler not to warn
about the unused debug locals.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200213081217.3107410-1-chris@chris-wilson.co.uk