drm/fb-helper: add fb_debug_* to DRM_FB_HELPER_DEFAULT_OPS

Add additional members fb_debug_enter and fb_debug_leave to helper
define. They are shared by all fb_ops implementations.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479078208-25221-3-git-send-email-contact@stefanchrist.eu