drm/msm/mdp4: Don't manage DSI PLL regulators in MDP driver

The MDP4 driver tries to request and set voltages for regulators required
by the DSI PLLs.

Firstly, the MDP4 driver shouldn't manage the DSI regulators, this should
be handled in the DSI driver. Secondly, it shouldn't try to set a fixed
voltage for regulators. Voltage constraints should be specified on the
regulator via DT and managed by the regulator core.

Remove all the DSI PLL regulator related code from the MDP4 driver. It's
managed in the DSI driver for MSM8960/APQ8064 already.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>