Apparently it's broken in the exact same way as the gmbus irq. For
reference of the full story see

commit c12aba5a
Author: Jiri Kosina <jkosina@suse.cz>
Date:   Tue Mar 19 09:56:57 2013 +0100

    drm/i915: stop using GMBUS IRQs on Gen4 chips

The effect is that we have a storm of unclaimed interrupts on the
legacy irq line. If that one is used by a different device then the
kernel will complain and rather quickly kill the irq source. Which
breaks any device trying to actually use the legacy irq line.

This regression has been introduced

commit 4aeebd74
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 31 09:53:36 2013 +0100

    drm/i915: dp aux irq support for g4x/vlv

Note that disabling MSI works around the issue, but we can't do that
since apparently then the hw will miss interrupts. At least if
relevant comments in i915_irq.c are accurate.

v2: Cross-reference dp aux and gmbus gen4 comments.

v3: Consolidate harder into i915_drv.h as suggested by Chris.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-tested-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>