drm/mgag200: Don't unpin the current cursor image's buffer.

Currently the displayed cursor buffer might be evicted from video memory.
Not unpinning the BO fixes this problem. At this point, pixels_current
also references the BO and it will be unpinned during the next cursor
update.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 94dc57b1 ("drm/mgag200: Rewrite cursor handling")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723075425.24028-4-tzimmermann@suse.de