drm/i915: Add a label for config DRM_I915_SPIN_REQUEST

If we don't give it a label, it does not appear as a configuration
option.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190612093111.11684-9-chris@chris-wilson.co.uk