drm/radeon: force dma32 to fix regression rs4xx,rs6xx,rs740

commit 4a2b6662c3632176b4fdf012243dd3751367bf1f upstream.

It seems some of those IGP dislike non dma32 page despite what
documentation says. Fix regression since we allowed non dma32
pages. It seems it only affect some revision of those IGP chips
as we don't know which one just force dma32 for all of them.

https://bugzilla.redhat.com/show_bug.cgi?id=785375

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>