msm: kgsl: Add NULL check for next_event hook

KGSL driver needs to check that the function pointer
is not NULL before calling the hook. next_event
is an optional function not implemented by the 2d
core. This fix ensures that optional and unimplemented
function will not invoked by the driver.

Change-Id: I687b028260e3ec304d973c6a5bded0a8fdbfb73b
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>