drm/mediatek: Stop iterating dma addresses when sg_dma_len() == 0

If dma_map_sg() merges pages when creating the mapping, only the first
entries will have a valid sg_dma_address() and sg_dma_len(), followed by
entries with sg_dma_len() == 0.

Signed-off-by: Anand K Mistry <amistry@google.com>
Signed-off-by: Anand K Mistry <amistry@chromium.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>