media: mtk_vcodec_util: Use dma_zalloc_coherent to replace dma_alloc_coherent + memset

dma_zalloc_coherent has implemented the dma_alloc_coherent() + memset(),
We prefer to dma_zalloc_coherent instead of open-codeing.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>