msm: vp: Clear APC_PLEVELx register's VREG_PD_EN bit

The current logic in the set_voltage is not clearing
VREG_PD_EN bit, so if this bit is set once then we will see
increase in the one PMIC step size from the given
voltage caller has requested to set.

Change-Id: Id91f0dbdec004bcc111efdcaa08e613240c191b6
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>