fbdev/radeon: use pci aperture helpers

It's not exactly the same since the open coded version doesn't set
primary correctly. But that's a bugfix, so shouldn't hurt really.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linux-fbdev@vger.kernel.org
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111154112.90575-7-daniel.vetter@ffwll.ch