drm/rockchip: Replace custom wait_for_vblanks with helper

Currently the driver uses a custom function to wait for flip to complete
after an atomic commit. It was needed before because of two problems:
 - there is no hardware vblank counter, so the original helper would
   have a race condition with the vblank interrupt,
 - the driver didn't support unreferencing cursor framebuffers
   asynchronously to the commit, which was what the helper expected.
Since both problems have been solved by previous patches, we can now
make the driver use the generic helper and remove custom waiting code.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>