drm/core: Preserve the fb id on close.

Keep the fb_id, which means that any application exiting without
unsetting the framebuffer from all planes will preserve its contents.

This is similar to preserving the initial framebuffer, except all
planes are preserved.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
[danvet: Remove unused variable, reported by Stephen Rothwell.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>