drm/i915: Mark up GTT sizes as u64

Since we use a 64b virtual GTT irrespective of the system, we want to
ensure that the GTT computations remains 64b even on 32b systems,
including treatment of huge virtual pages.

No code generation changes on 64b:

Reported-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108282
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025091823.20571-1-chris@chris-wilson.co.uk
(cherry picked from commit 9125963a

)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>