msm: kgsl: Do not return invalid power stats when the device is off

adreno_power_stats() was returning unitialized data when the device
state was not active. Leave early from the function if the device
is not active and return 0s instead.

Change-Id: Ic0dedbadf6201cadb4472327c4ce7f9d9d32b82d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>