Fix a pointer cast warning in the SIS DRM code.

This was introduced in patch ce65a44d.
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>