drm/rockchip: vop: fix iommu crash with async atomic

After async atomic_commit callback, drm_atomic_clean_old_fb will
clean all old fb, but because async, the old fb may be also on
the vop hardware, dma will access the old fb buffer, clean old
fb will cause iommu page fault.

Reference the fb and unreference it when the fb actuall swap out
from vop hardware.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>