drm/armada: move overlay plane work out from under spinlock

Move the overlay plane work out from under the spinlock so that both the
primary and overlay planes run their work in the same context.  This is
necessary so that we can use frame works with the overlay plane.

However, we must update the CRTC registers under the spinlock, so fix up
the overlay code for that.

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