drm/msm/dsi: Update source PLL selection in DSI PHY

The source PLL to be used by each DSI PHY should be decided by
DSI manager based on dual DSI information, while the register
programming to select PLL is different from one type of PHY to
another. This change adds the H/W difference to PHY configuration
and updates the interface between DSI manager and PHY.

With this change, PLL selection can be supported on different
targets.

Change-Id: I0dbe7c4798a8aaead5a7e541abd6cb5e95db0328
Signed-off-by: Hai Li <hali@codeaurora.org>