drm: don't cast a pointer to pointer of list_head

The casting is safe only when the list_head member is the first member of
the structure.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>