drm/i915: Simplify relocate_entry_gtt() and make 64-bit safe

Even though we should not try to use 4+GiB GTTs on 32-bit systems, by
using a local variable we can future proof the code whilst making it
easier to read.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Appease checkpatch a bit.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>