msm: mdss: use LDO mode in DSI PHY on 8x10

DSI PHY supports two regulator modes: LDO and DCDC.
While DCDC offers power efficiency, it requires an
external inductor which adds to the BOM cost.  On
8x10, it is required to use LDO mode to save the
extra cost.

Change-Id: I667ee00b78c916bcdd5096a1b72dce863ec0880c
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>