msm: mdss: support 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. So it is required to enable LDO mode
to save the extra PCB board cost.

Change-Id: I156ba8af7857743db5373a9e3f79d948cf9e1427
Signed-off-by: Mao Flynn <Flynn@codeaurora.org>