drm/amdkfd: check for null dev to avoid a null pointer dereference

The call to kfd_device_by_id can potentially return null, so check that
dev is null and return with -EINVAL to avoid a null pointer dereference.

Detected by CoverityScan CID#1454629 ("Dereference null return value")

Fixes: 5d71dbc3a588 ("drm/amdkfd: Implement image tiling mode support v2")
Signed-off-by: Colin Ian King <[e-mail]>
Signed-off-by: Oded Gabbay <[e-mail]>

drivers/gpu/drm/amd/amdkfd/kfd_chardev.c

···
 	int err = 0;
 
 	dev = kfd_device_by_id(args->gpu_id);
+	if (!dev)
+		return -EINVAL;
 
 	dev->kfd2kgd->get_tile_config(dev->kgd, &config);