drm/fb-helper: add DRM_FB_HELPER_DEFAULT_OPS for fb_ops

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 <[email protected]>
Signed-off-by: Stefan Christ <[email protected]>
Reviewed-by: Gustavo Padovan <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]