[media] v4l: vsp1: Use DFE instead of FRE for frame end

The DFE and FRE interrupts are both fired at frame completion, as each
display list processes a single frame. This won't be true anymore when
using image partitioning, switch to DFE in preparation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran+renesas@bingham.xyz>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>