drm/armada: convert page_flip to use primary plane atomic_update()

page_flip requests happen asynchronously, so we can't wait on the
vblank event before returning to userspace, as the transitional plane
update helper would do.  Craft our own implementation that keeps the
asynchronous behaviour of this request, while making use of the atomic
infrastructure for the primary plane update.

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