For VCE to work properly the start of the GTT space must be aligned to a
4GB boundary.

v2: add comment why we do this
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>