drm/ast: Don't call ast_show_cursor() from ast_cursor_move()

Separating the cursor's move() function from the show() function in
preparation of further rework of the cursor update code.

'Showing' the cursor from within the move() function is required to
update the cursor position.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927090309.10254-2-tzimmermann@suse.de