[media] omap_vout: use omapdss's version instead of cpu_is_*

cpu_is_* class functions create a dependency to OMAP platform code.
omapdss driver, which omap_vout uses, exposes a function to get the
version of the DSS hardware.

To remove the dependency to OMAP platform code this patch changes
omap_vout to use the omapdss version. For most of the checks, the ones
dealing with DSS differences, this is actually more correct than using
cpu_is_* functions. For the check whether VRFB is available or not this
is not really correct, but still works fine.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>