drm/radeon/kms: Disable agp only if we are dealing with an AGP GPU

On IGP if you pass option agpmode=-1 you would overwrite the set_page
function callback with improper function which endup in non functioning
hw. This patch will disable agp when giving agpmode=-1 parameter only
if we are dealing with an AGP GPU.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>