sunxi: video: Use frontend for dma on sun4i to fix memory bandwidth problems

Testing has shown that on sun4i the display backend engine does not have
deep enough fifo-s causing flickering / tearing in full-hd mode due to
fifo underruns. On sun4i use the display frontend engine to do the dma from
memory, as the frontend does have deep enough fifo-s.

As added advantage of this is that it results in much better memory bandwidth
as it reduces the amount of dram bank switches, for more details see:

http://ssvb.github.io/2014/11/11/revisiting-fullhd-x11-desktop-performance-of-the-allwinner-a10.html

Note that this changes the pipeline searched for in the simplefb node, we can
get away with doing this now, since no kernel has yet shipped with simplefb
dtb nodes, and I will make sure to get a simplefb node with the new pipeline
into 3.19 before it ships.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>

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