drm/core: Preserve the framebuffer after removing it.

Previously RMFB and fd close chose to disable any plane that had
an active framebuffer from this file. If it was a primary plane the
crtc was disabled. However the fbdev code or any system compositor
should restore the planes anyway so there's no need to do it twice.

The old fb_id is zero'd, so there's no danger of being able to
restore the fb from fb_id.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>