msm: kgsl: Disable power counters before slumber

If the power counter is on, the GMU will report that
it is busy. Disable the power counter before calling
the slumber OOB to make sure it will be idle. Then
reenable the power counter when coming out of slumber.

Also make GMU error messages for idle and OOBs
more descriptive.

CRs-Fixed: 2017390
Change-Id: I47300e48c4c8ca10779344e826f80b0249c7571e
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>