drm: mediatek: use common helper for extracting pages array

Use common helper for converting a sg_table object into struct
page pointer array.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>