drm/fb-helper: streamline drm_fb_helper_single_fb_probe

No need to check whether we've allocated a new fb since we're not
always doing that. Also, we always need to register the fbdev and add
it to the panic notifier.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>