drm/ast: Move cursor pageflip into helper

The new helper ast_cursor_page_flip() switches the cursor's front and
back BOs. This simplifies the cursor plane's update helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-6-tzimmermann@suse.de