amdgpu: use EINVAL instead of EBADMSG in amdgpu_bo_cpu_unmap()

EBADMSG is a streams errno.  OpenBSD does not implement streams and does
include the streams errnos, this commit fixes the build on OpenBSD.

None of the callers of this function check the return value for -EBADMSG.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>