msm: kgsl: Do not delete uninitialized preempt timer

Deleting uninitialized timer can result to warning message.
Add a proper check to skip uninitialized preempt timer.

Change-Id: I3c180435314bb7e52d3ffb2c0934fbb883c3a079
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>