drm/i915: Remove 64b mmio write vfuncs

We don't have safe 64-bit mmio writes as they are really split into
2x32-bit writes. This tearing is dangerous as the hardware *will*
operate on the intermediate value, requiring great care when assigning.
(See, for example, i965_write_fence_reg.) As such we don't currently use
them and strongly advise not to us them. Go one step further and remove
the 64-bit write vfuncs.

v2: Add some more details to the comment about why WRITE64 is absent,
and why you need to think twice before using READ64.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160906144538.4204-1-chris@chris-wilson.co.uk