drm/msm/dsi: Pass down use case to PHY

For some new types of DSI PHY, more settings depend on
use cases controlled by DSI manager. This change allows
DSI manager to setup PHY with a use case.

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