msm: kgsl: devfreq_target requests can happen during SLUMBER

Bus updates should be ignored if the bus is already off.  The bus
will wake at the default frequency for the active power level.

Change-Id: I63b8d1aacfe45971929f21f7c63bdd8ba16536e1
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>