tests/amdgpu: Fix TMZ secure bounce test

Fix the TMZ secure bounce test, in that Bob's
buffer has to be created encrypted (with the
encrypted flag set), so that when we copy from
Alice's buffer, which is also encrypted, to Bob's
buffer, the copy can be successful and the data
actually copied.

This fixes the test and it no longer fails. Tested
on Sienna Cichlid.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Aaron Liu <aaron.liu@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <Alexander.Deucher@amd.com>