drm/mediatek: init RDMA with valid buffer

RDMA is initialized with buffer pointer 0. This creates IOMMU errors
when the DMA try to read this unmapped address.
Instead of initializing with 0, initialize it with some allocated
buffer address.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Change-Id: I98162ce1fe1b08eb602fce577bb0f146259b5f86