drm/msm/dsi: Separate PHY to another platform device

There are different types of PHY from one chipset to another, while
the DSI host controller is relatively consistent across platforms.
Also, the PLL inside PHY is providing the source of DSI byte and
pixel clocks, which are used by DSI host controller. Separated devices
for clock provider and clock consumer make DSI driver better fit into
common clock framework.

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