drm/rockchip: Unreference framebuffers from flip work

Currently the driver waits for vblank and then unreferences old
framebuffers from atomic commit code path. This is however breaking the
legacy cursor API, which requires the updates to be fully asynchronous.
Instead of just adding a special case for cursor, we can have actually
smaller amount of code to unreference any changed framebuffer from a
flip work.

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