drm/i915: Include a note about the dangers of I915_READ64/I915_WRITE64

It is important that the user is fully aware that the seemingly atomic
read/write of a 64-bit value from MMIO space, may in fact be 2 separate
operations of 32-bits. This can lead to hilarity, such as

commit d18b9619034230b6f945e215276425636ca401fe
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Jul 10 13:36:23 2013 +0100

    drm/i915: Fix incoherence with fence updates on Sandybridge+

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -2736,6 +2736,12 @@ void vlv_force_wake_put(struct drm_i915_private *dev_priv, int fw_engine);
 #define I915_READ_NOTRACE(reg)		dev_priv->uncore.funcs.mmio_readl(dev_priv, (reg), false)
 #define I915_WRITE_NOTRACE(reg, val)	dev_priv->uncore.funcs.mmio_writel(dev_priv, (reg), (val), false)
 
+/* Be very careful with read/write 64-bit values. On 32-bit machines, they
+ * will be implemented using 2 32-bit writes in an arbitrary order with
+ * an arbitrary delay between them. This can cause the hardware to
+ * act upon the intermediate value, possibly leading to corruption and
+ * machine death. You have been warned.
+ */
 #define I915_WRITE64(reg, val)	dev_priv->uncore.funcs.mmio_writeq(dev_priv, (reg), (val), true)
 #define I915_READ64(reg)	dev_priv->uncore.funcs.mmio_readq(dev_priv, (reg), true)