HACK: OMAP: DSS2: VENC: disable VENC on OMAP4 to prevent crash

Something seems to be wrong with OMAP4 & VENC, and register access fails
in omap_venchw_probe().

This patch skips venc driver registration on OMAP4, thus circumventing
the problem for now.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>