media: vsp1: Simplify DRM UIF handling

In commit 6732f313 ("media: v4l: vsp1: Fix uif null pointer access")
the handling of the UIF was over complicated, and the patch applied
before review.

Simplify it to keep the conditionals small.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>