drm/ast: Init cursors before creating modesetting structures

The cursor helpers reserve buffer objects in VRAM and update their
content. So although tied to modesetting, cursor helpers are more
of a memory manager. The modesetting's cursor plane requires this
functionality, so initialize cursors before modesetting.

While at it, also add an error check for ast_cursor_init().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-12-tzimmermann@suse.de