media: v4l: vsp1: Fix uif null pointer access

RZ/G2L SoC has no UIF. This patch fixes null pointer access, when UIF
module is not used.

Fixes: 5e824f98("media: v4l: vsp1: Integrate DISCOM in display pipeline")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>