drm/amdkfd: Fix the error return code in kfd_ioctl_unmap_memory_from_gpu()

Passing NULL pointer to PTR_ERR will result in return value of 0
indicating success which is clearly not what it is intended here.
This patch returns -EINVAL instead.

v2: change ret code to -ENODEV

Fixes: 5ec7e028 ("drm/amdkfd: Add ioctls for GPUVM memory management")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>