drm/mediatek: mtk_drm_plane: Use kmalloc in mtk_plane_duplicate_state

There is no need to zero out the newly allocated memory because we are
duplicating all members of struct mtk_plane_state: switch to kmalloc
to save some overhead.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220118133759.112458-1-angelogioacchino.delregno@collabora.com/


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>