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 d18b9619
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>