drm/amdkfd: fix kernel crash on releasing NULL sysfs entry

drm/amdkfd: fix kernel crash on releasing NULL sysfs entry

there is an abnormal case that When a process re-opens kfd
with different mm_struct(execve() called by user), the
allocated p->kobj will be freed, but missed setting it to NULL,
that will cause sysfs/kernel crash with NULL pointers in p->kobj
on kfd_process_remove_sysfs() when releasing process, and the
similar error on kfd_procfs_del_queue() as well.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>