drm/i915: Limit the auto arming of mmio debugs on vlv/chv

The capability to detect unclaimed register access was
recently introduced for vlv/chv platforms. Apparently
there are plenty of unclaimed access on these platforms,
resulting in new dmesg warns. But as we are trying to form
a beachhead for CI/Bat, all new warns are adding to the
noise and thus not desirable at this point in time.

Make it so that if in these platforms the automatic arming
was responsible for mmio_debug enabling, ignore the warns.

If user/dev wants to fix these, he can still do so by
i915.mmio_debug=1234.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1453285943-24614-1-git-send-email-mika.kuoppala@intel.com