drm/i915/selftests: Ensure we don't clamp a random offset to 32b

Specify that we do want a 64b value for sizeof(u32) as we want to
compute the mask of the upper 62bits.

v2: Use round_down() for automatic type promotion

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710161413.7115-1-chris@chris-wilson.co.uk