msm: kgsl: Use proper check for idle

The sequence to check for GMU idle is changed.
The new sequence is to check GPUBusyIgnAHB, then
do notify slumber, then check CXGXCPUBusyIgnAHB.
Also check for nonzero error case instead of 0
when calling wait_for_gmu_idle().

Additionally, check for the proper slumber value and
write a register to make GMU not busy before reading
status. Also remove a redundant slumber check.

CRs-Fixed: 2017390
Change-Id: I5ad8f7af941ebe08f9ce41ea9c68eae0c8d3c47b
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>