drm/radeon: fix page flipping hangs on r300/r400

We've been getting reports of complete system lockups with rv3xx hw on
AGP and PCIE when running gnome-shell or kwin with compositing.

It appears the hw really doesn't like setting these registers while
stuff is running, this moves the setting of the registers into the modeset
since they aren't required to be changed anywhere else.

fixes: https://bugs.freedesktop.org/show_bug.cgi?id=35183

Reported-and-tested-by: Álmos <aaalmosss@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>