drm/i915: Restrict L3 remapping sysfs interface to dwords

The L3 cache remapping is stored as u32 elements, and we should ensure
that the user only supplies complete slice information(u32).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191004105958.1741-1-chris@chris-wilson.co.uk