drm/amdkfd: Check debug trap enable before write dbg_ev_file

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.

Signed-off-by: Lin.Cao <lincao12@amd.com>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>