[media] omap_vout: Fix compile error in 3.1

This patch is against the mainline v3.1 release (c3b92c87) and
fixes a compile error when building for OMAP3+DSS+VOUT

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Gary Thomas <gary@mlbassoc.com>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>