drm/armada: implement primary plane update

Implement primary plane update without having to go through a modeset
to achieve that; the hardware does not require such complexity.  This
means we treat the primary plane as any other, allowing the format,
size, position and scaling to be updated via the normal plane ioctls.

This also allows us to seemlessly disable and re-enable the primary
plane when (eg) displaying full-frame video without any graphic
clipping the overlaid video - which saves wasting memory bandwidth
needlessly verifying that the colorkey is indeed filling the entire
primary plane.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>