drm/radeon/kms: fix agp mode setup on cards that use pcie bridges

Asics that use an AGP to PCIE bridge don't have the AGP_STATUS
register so just use whatever mode the host side setup.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: Jerome Glisse <glisse@freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>