drm: WARN when removing unallocated node

drm: WARN when removing unallocated node

The conditional is usually a recoverable driver bug, and so WARNing, and
preventing the drm_mm code from doing potential damage (BUG) is
desirable.

This issue was hit and fixed twice while developing the i915 multiple
address space code. The first fix is the patch just before this, and is
hit on an not frequently occuring error path. Another was fixed during
patch iteration, so it's hard to see from the patch:

commit c6cfb325677ea6305fb19acf3a4d14ea267f923e
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Fri Jul 5 14:41:06 2013 -0700

    drm/i915: Embed drm_mm_node in i915 gem obj

From the intel-gfx mailing list, we discussed this:
References: <20130705191235.GA3057@bwidawsk.net>

Cc: Dave Airlie <airlied@redhat.com>
CC: <dri-devel@lists.freedesktop.org>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>