drm/radeon: forever loop on error in radeon_do_test_moves()

The error path does this:

	for (--i; i >= 0; --i) {

which is a forever loop because "i" is unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org