drm/i915: don't warn if IRQs are disabled when shutting down display IRQs

This was always the case on our suspend path, but it was recently
exposed by the change to use our runtime IRQ disable routine rather than
the full DRM IRQ disable.  Keep the warning on the enable side, as that
really would indicate a bug.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>