drm/msm/edp: Drop regulator_set_voltage call

The eDP driver tries to set a fixed voltage for one of its regulators(vdda)
before enabling it. This shouldn't be done by the driver, the voltage
constraints should be specified on the regulator via DT and managed by
the regulator core. A driver should call regulator_set_voltage only if
it needs to change the voltage during runtime. Drop the
regulator_set_voltage call. Mention in a comment the voltage that the
regulator expects.

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