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.

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