amdgpu: a NULL ->mm does not mean a thread is a kthread

amdgpu: a NULL ->mm does not mean a thread is a kthread

Use the proper API instead.

Fixes: 70539bd795002 ("drm/amd: Update MEC HQD loading code for KFD")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: http://lkml.kernel.org/r/20200404094101.672954-1-hch@lst.de
Link: http://lkml.kernel.org/r/20200404094101.672954-2-hch@lst.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>