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: 5d71dbc3 ("drm/amdkfd: Implement image tiling mode support v2")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>

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

@@ -892,6 +892,8 @@ static int kfd_ioctl_get_tile_config(struct file *filep,
 	int err = 0;
 
 	dev = kfd_device_by_id(args->gpu_id);
+	if (!dev)
+		return -EINVAL;
 
 	dev->kfd2kgd->get_tile_config(dev->kgd, &config);