drm/i915: Compare user's 64b GTT offset even on 32b

Beware mixing unsigned long constants and 64b values, as on 32b the
constant will be zero extended and discard the high 32b when used as
a mask!

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/20181025091823.20571-2-chris@chris-wilson.co.uk