msm: kgsl: Don't call through pwrscale functions when entering SLUMBER

No additional calls to pwrscale idle or busy should be allowed when
transitioning to SLUMBER state.

Change-Id: I9488569758d263752c9778b8603f8e80bbc41ea1
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>