drm/i915: Fix restore of 965 fence regs since the register tracing change.

We were reading our 64-bit value in I915_READ64 and returning 32 bits
of it.  The restoration of fence regs at resume then had a zero end
value, and the fence had no effect.

Version 2: Split register access functions into per-size versions

Sharing code between different sizes seemed reasonable when we only
needed a single copy, but as 64-bit access requires its own version,
it makes sense to just split them out for each size.

Reported-by: Peter Clifton <pcjc2@cam.ac.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Keith Packard <keithp@keithp.com>
[ickle: use a macro to create the various read/write routines]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>