drm/i915: Make the ring IMR handling private

As the IMR for the USER interrupts are not modified elsewhere, we can
separate the spinlock used for these from that of hpd and pipestats.
Those two IMR are manipulated under an IRQ and so need heavier locking.

Reported-and-tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 6b1d9a5..be9087e 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -55,6 +55,7 @@
 	int		effective_size;
 	struct intel_hw_status_page status_page;
 
+	spinlock_t	irq_lock;
 	u32		irq_refcount;
 	u32		irq_mask;
 	u32		irq_seqno;		/* last seq seem at irq time */