drm/fb-helper: add DRM_FB_HELPER_DEFAULT_OPS for fb_ops

The define DRM_FB_HELPER_DEFAULT_OPS provides the drm_fb_helper default
implementations for functions in struct fb_ops. A drm driver can use it
like:

    static struct fb_ops drm_fbdev_cma_ops = {
        .owner          = THIS_MODULE,
        DRM_FB_HELPER_DEFAULT_OPS,
        /* driver specific implementations */
    };

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1475182136-15191-2-git-send-email-contact@stefanchrist.eu