msm: cpr: Use regulator_get_voltage to get the voltage at PMIC

CPR driver was caching the last set voltage into the calibrated_uV,
which is wrong in the current configuration since the another entity
like DCVS driver could change the voltage by directly setting the
GO bit and appropriate performance level which could be different
from what was set by last regulator_set_voltage. Due to this
CPR recommendations could get adjusted at the wrong voltage. So
always the read the voltage through the regulator_get_voltage
API to get the voltage at the PMIC, rather than caching the values
in the CPR driver.

Change-Id: I2ad8efc36d7dc0f647b2052985b57e941a993b7e
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>