drm/radeon: Change the default to PCI on PowerPC

AGP mode is unstable on PowerPC. Symptoms are generally of the form:

[ 1228.795711] radeon 0000:00:10.0: ring 0 stalled for more than 10240msec

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