SiS DRM: fix a pointer cast warning

Fix a pointer cast warning in the SIS DRM code.

This was introduced in patch ce65a44d

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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>