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.

Signed-off-by: QintaoShen <unSimple1993@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>