msm: mdss: enable LDO mode for DSI PHY regulator

DSI PHY supports two regulator modes: LDO and DCDC.
While DCDC offers power efficiency, but it requires
an external inductor. On skuh, it is required to
enable LDO mode to save the extra PCB board cost.

Change-Id: I90cce537a7ca21d571a27e888df7d0d00ad89c49
Signed-off-by: Mao Flynn <Flynn@codeaurora.org>