drm/armada: disable planes at next blanking period

Disable planes at the next blanking period rather than immediately.
In order to achieve this, we need to delay the clearing of dcrtc->plane
until after the next blanking period, so move that into a separate
work function.  To avoid races, we also need to move its assignment in
the overlay code.

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