drm/mgag200: Add init and fini functions for cursor handling

Moving the cursor initialization and cleanup into separate functions
makes the overall code slightly more readable.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927091301.10574-3-tzimmermann@suse.de