drm/ast: Remove obsolete or unused cursor state

The ast driver's data structures store unused or uncecessary cursor
state. Most of the cursor state is already stored elsewhere and can
be retrieved when necessary. Remove the obsolete fields and adapt
users accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-4-tzimmermann@suse.de