msm: kgsl: Update idle checks to match GMU FW 0.6

GMU FW 0.6 changed some behavior during prepare for slumber
that affects checking for idle. Update the KMD to work with
this.

CRs-Fixed: 2017390
Change-Id: I1ffc820f5531e978c9221f1d345a3ed860e5bd59
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>