When the display start interrupt occurs, we know that the hardware has
finished loading the active display list. The driver then proceeds to
recycle the list, assuming it won't be needed anymore.

This assumption holds true for headerless display lists, as the VSP
doesn't reload the list for the next frame if it hasn't changed.
However, this isn't true anymore for header display lists, as they are
loaded at every frame start regardless of whether they have been
updated.

To prepare for header display lists usage in display pipelines, we need
to postpone recycling the list until it gets replaced by a new one
through a page flip. The driver already does so in the frame end
interrupt handler, so all we need is to skip list recycling in the
display start interrupt handler.

While the active list can be recycled at display start for headerless
display lists, there's no real harm in postponing that to the frame end
interrupt handler in all cases. This simplifies interrupt handling as we
don't need to process the display start interrupt anymore.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>