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 mdp driver and control
the voltage to avoid frame shifting.

Change-Id: I7ec9e29a4972373775406fa29a5037fd43fef315
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>