drm/mgag200: Allocate cursor BOs at high end of video memory

By putting cursor BOs at the high end of the video memory, we can avoid
memory fragmentation. Starting at the low end, contiguous video memory is
available for framebuffers.

The patch also simplifies the buffer swapping and aligns it with the
ast driver. If there are more drivers with similar requirements, the
code could be moved into a shared place.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927091301.10574-8-tzimmermann@suse.de