msm: kgsl: Add NULL check for preemption_schedule call

preemption_schedule() is not implemented for A3xx targets.
Invoking this function without NULL check result into crash
in A3xx target. Fix this by adding a NULL check before invoking
the preemption_schedule().

Change-Id: Ic600235f149cade57fedc5454bdc0f6794c67bd9
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>