video: rockchip: fb: disable ion_unmap_iommu video buffer

video: rockchip: fb: disable ion_unmap_iommu video buffer

The mapping action of ion_map_iommu will cost longer time
and more cpu load if mapped buffer size is large, especially
for 4K video buffer.

If this buffer has been mapped, ion_map_iommu will use the
mapped table before, almost no time consuming. And the mapping
relationship will be free when this buffer is free.

So we disable ion_unmap_iommu to reduce RK_FBIOSET_CONFIG_DONE
cost time and cpu load. On 3228, surfaceflinger cpu load reduce
to 4% from 9% when playing 4K video.

Change-Id: I64b2d4550b21b6ff44d8cceafe114ca8405601d9
Signed-off-by: Zheng Yang <zhengyang@rock-chips.com>