drm: omapdrm: venc: Don't export omap_dss_pal_vm and omap_dss_ntsc_vm

The two variables are never used outside of their compilation unit, make
them static.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>