regulator: qcom_pm8008: correct parent supply voltage voting

Make a request for the parent supply voltage (with headroom
included) inside of pm8008_regulator_enable().  This ensures that
the required headroom is enforced if a consumer calls
regulator_disable() followed by regulator_enable() for one of
the PM8008 LDO regulators.

Also, change the procedure used to request the parent supply
voltage inside of pm8008_regulator_set_voltage().  The parent
voltage request should only be made if the PM8008 LDO is enabled.
Additionally, the request should be made after PM8008 LDO
voltage is changed if the voltage is being reduced.  This ensures
that there are no transient headroom voltage violations.

Change-Id: Ic008689e3cb6361265767fe7daa36cf4d14ae0f3
Signed-off-by: David Collins <collinsd@codeaurora.org>