drm/msm/dsi: Rename *dual panel* to *dual DSI*

The current term of *dual panel* in DSI driver code causes confusion.
It is supposed to indicate the panel using two DSI links. Rename it
to *dual DSI*.

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>