drm/amdgpu: fix the wrong logic checking when secure buffer is created (v3)

While the current amdgpu doesn't support TMZ, it will return the error if user
mode would like to allocate secure buffer.

v2: we didn't need this checking anymore.
v3: only print message once time.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Nirmoy Das <Nirmoy.Das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>