combine VPCONFIG init voltage setup into common function and use from
both vp_enable and from vp_forceupdate_scale().

NOTE: this patch changes the sequence of when the initVDD bit is
cleared.  The bit is now cleared immediately after it was written.
Since only the rising edge of this bit has any affect according to the
TRM, the exact timing of clearing of this bit should not have any
effect.
Signed-off-by: Kevin Hilman <khilman@ti.com>