drm: rcar-du: Only initialise TVM_TVSYNC mode when supported

The R-Car DU as found on the D3, E3, and V3U do not have support
for an external synchronisation method.

In these cases, the dsysr cached register should not be initialised
in DSYSR_TVM_TVSYNC, but instead should be left clear to configure as
DSYSR_TVM_MASTER by default.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>