drm/mgag200: Move cursor-image update to mgag200_show_cursor()

Separating the management of buffer objects from updating the hardware
cursor buffer gives the code more structure. While doing this, we can
further split the image-update code into code for writing the buffer,
setting the base scan-out address, and enabling the cursor. The first
two operations are in dedicated functions update() and set_base().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927091301.10574-5-tzimmermann@suse.de