drm/ast: Move ast_{show,hide}_cursor() within source file

This patch only moves around code for easier review of later patches. No
functional cahnges are made.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927090309.10254-3-tzimmermann@suse.de