drm/mgag200: Move cursor BO swapping into mgag200_show_cursor()

Selecting the correct BO for the new cursor image is not relevant
outside of mgag200_show_cursor(). Let the function do the work.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927091301.10574-6-tzimmermann@suse.de