drm/i915: Move low-level swizzling code to i915_gem_fence.c

It fits more with the low-level fence code, and this move leaves only
the userspace tiling ioctl handling in i915_gem_tiling.c.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
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>