msm: mdss: Configure minimum voltage for CX power rail

For smart panels, we observe abnormal waveforms and
unexpected voltage spikes on DSI data lanes when the cx
power rail goes to minimum voltage. Vote for the cx
regulator in mdp3 driver and control the voltage to
avoid frame shifting.

Change-Id: I8ce8d7621f903966f519da563a9c57701146be24
Signed-off-by: Shivaraj Shetty <shivaraj@codeaurora.org>