drm/radeon: disable AGP by default

Always use the PCI GART instead. We just have to many cases
where AGP still causes problems. This means a performance
regression for some GPUs, but also a bug fix for some others.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>