drm/amdgpu: remove the alignment placeholder for secure buffer

The alignment should match the page size for secure buffer, so we didn't
configure it anymore.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>