msm: clock: Call regulator_enable/disable when updating vdd

When all clocks on a regulator are disabled, regulator_disable should
be called.  Likewise, when first clock is prepared, regulator_enable
should be called.

Add regulator_enable/disable function during voltage voting.

Change-Id: I791e6faffe58870990a7865b7f241ec53f50012b
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>