MC_VM_AGP_* registers should not be programmed by guest driver.

v2: move early return outside of loop

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Samir Dhume <samir.dhume@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>