drm/i915/gvt: Use offsetofend() rather than offsetof + sizeof

Compute the offset of the end of the crc32 field using offsetofend()
rather than open-coding.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>