drm/mediatek: Update the definition of connection from RDMA1 to DPI0

This patch update the definition of connection from RDMA1 to DPI0.
Change the term MOUT to SOUT.

Because our HW datasheet use the term SOUT to match its function for RDMA.
For consistency, changing the name from MOUT to SOUT is better.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>