drm/i915: kerneldoc for tiling IOCTL and swizzle functions

Chris rightfully suggested that documenting fences without documenting
the BO tiling tracking doesn't make much sense, so fix that.

The important bit to stress here (since it lead to some confusion) is
the GEM doesn't really care about tiling. Except for a few select cases
where the kernel needs to manage something that userspace can't take
care of: Namely the limited number of fences and fixing up swizzling,
although we still fail at the later.

v2: Move the low-level tiling/swizzling functions and kerneldoc to
i915_gem_fence.c and leave only the userspace interface here.
Suggested by Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>