msm: kgsl: Enable gfx rail retention voltage

When the gpu is sufficiently idle, set the gfx rail voltage to
retention to save power. Leave a vote on the MX rail to avoid
RPM transaction.

Change-Id: If6b49aee49d64bd57aa9c4cce153017af3f8eae6
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>