drm/ast: Add helper to hide cursor

As the inverse to ast_cursor_show(), ast_cursor_hide() disables the
HW cursor.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-9-tzimmermann@suse.de