msm: kgsl: Make sure the bus is on to consider power enabled

In theory only reg reads/writes should be using this check, and
they don't need the bus.  But just in case, make sure it is on
before allowing any activity.

Change-Id: I660fb947ad16e2872cfb95a9d99200a197b37f31
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>