msm: kgsl: Set the GX rail voltage before enabling it

Make sure the rail turns on with a consistent voltage.  Also
this sequence moves an RPM call from after the rail is on to
before it is powered up which saves a bit of power.

Change-Id: Ided5f9777879fa48fd594c84b31cec9cdf513098
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>