drm/i915: Protect gen7 irq_seqno_barrier with uncore lock

Faced with sporadic machine hangs on gen7, that mimic the issue of
concurrent writes to the same cacheline and seem to start with
commit 9b9ed309 (drm/i915: Remove forcewake dance from seqno/irq
barrier on legacy gen6+), let us restore the spinlock around the mmio
read.

Fixes: 9b9ed309

 (drm/i915: Remove forcewake dance from seqno/irq...)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461744121-27051-1-git-send-email-chris@chris-wilson.co.uk
Tested-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>