msm: kgsl: use the same logic in adreno_idle and adreno_isidle

These two functions need to agree on the meaning of "idle",
so that calling adreno_isidle() right after an adreno_idle()
will always return true.

Change-Id: I7cddf73773186c3ec8b56c111affacac3b07fcc7
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>