drm/amdkfd: Check debug trap enable before write dbg_ev_file

In the Linux kernel, the following vulnerability has been resolved: drm/amdkfd: Check debug trap enable before write dbg_ev_file In interrupt context, write dbg_ev_file will be run by work queue. It will cause write dbg_ev_file execution after debug_trap_disable, which will cause NULL pointer access. v2: cancel work "debug_event_workarea" before set dbg_ev_file as NULL.