msm: mdss: update voltage level configuration for CX supply

Read the minimum and the maximum voltage levels for the CX supply
from the device tree and use those values to program the CX supply.
This allows for flexibility in configuring the levels across
multiple targets.

CRs-Fixed: 1053687
Change-Id: Ib704bfa6c6ca9fc3d90ab76a2e4aef02dc48822a
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>