drm/i915: Use preferred kernel types in i915_gem_gtt.c

Make checkpatch happy and make the use of u32/u64 consistent throughout
i915_gem_gtt.[ch]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-23-chris@chris-wilson.co.uk