msm: mdss: add support to configure svs voltage on cx rail

For dsi panels, we observe display fading issue
on few of the existing hardware version when the
cx power rail goes to svs voltage. Add support to
vote for the cx regulator in mdp driver if needed
and control the voltage to avoid any
display fading issues.

Change-Id: I0ea98d46ec1464b9311fe2a5364499e27975651f
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>