drm: rcar-du: Remove an unneeded NULL check

"params" can't be NULL here.  The next lines assume that we either
hit the break statement of "params->mpixelclock == ~0UL".  The
inconsistent NULL checking makes static checkers complain.  I've just
removed the test.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>