driver: gpu: drm: i915: remove cast for kzalloc return value

remove cast for kzalloc return value.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>