drm/amdkfd: Check for potential null return of kmalloc_array()

In the Linux kernel, the following vulnerability has been resolved:

drm/amdkfd: Check for potential null return of kmalloc_array()

As the kmalloc_array() may return null, the 'event_waiters[i].wait' would lead to null-pointer dereference.

Therefore, it is better to check the return value of kmalloc_array() to avoid this confusion.