max range of the agp aperture is 48 bits, so
enlarge agp_start address into 48bit with all bits set
Signed-off-by: Frank.Min <Frank.Min@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>