The overlay plane support updates asynchronously to the request, but the
drm_plane_helper_update() transitional helper waits for a vblank event
before releasing the framebuffer.  Using the transitional helper would
make the call block, which would introduce a performance regression.

Convert the overlay plane update to use the atomic state structures and
methods for the plane, but implement our own legacy update method
rather than the transitional helper.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>