drm: rcar-du: Track dma-buf fences

We have to check dma-buf reservation objects of our framebuffers before
we use them. Otherwise, another driver might be writing on the same
buffer which we are using. This would cause visible tearing effects
on display.

We can use existing atomic helper functions to solve this problem.

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>