gpu: ipu-v3: ipu-dmfc: Use static DMFC FIFO allocation mechanism

For all video modes we support currently, we always get 2 slots for
a plane by using the current existing dynamic DMFC FIFO allocation
mechanism.  So, let's change to use the static one to simplify the
code.  This also makes it easier to implement the atomic mode setting
as we don't need to handle allocation failure cases then.

Signed-off-by: Liu Ying <gnuiyl@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>