drm/armada: fix plane location and size for interlace

When the CRTC is programmed for interlace, we have to halve the Y
parameters for the plane.  Rather than doing this in the update
function (which would need the calculation repeated for the old
state as well as the new state), arrange to do the calculation in
atomic_check and save it in our private plane state structure.

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