drm/imx: use PRG/PRE when possible

Allow the planes to use the PRG/PRE units as linear prefetchers when
possible. This improves DRAM efficiency a bit and reduces the chance
for display underflow when the memory subsystem is under load.

This does not yet support scanning out tiled buffers directly, as this
needs more work, but it already wires up the basic interaction between
imx-drm, the IPUv3 driver and the PRG and PRE drivers.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>