drm/fb-helper: Handle function NULL argument

Make functions tolerate that the drm_fb_helper argument is NULL.
This is useful for drivers that continue probing when fbdev emulation
fails and not having to do this check themselves.
Update docs for functions that already handles this.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171030153951.56269-2-noralf@tronnes.org