drm/i915: Don't BUG_ON in i915_gem_obj_offset

drm/i915: Don't BUG_ON in i915_gem_obj_offset

A WARN_ON is perfectly fine.

The BUG in here seems to be the cause behind hard-hangs when I cat the
i915_gem_pageflip debugfs file (which calls this from an irq
spinlock). But only while running a full igt run after a while. I
still need to root cause the underlying issue.

I'll also start reject patches which add new BUG_ON but don't come
with a really good justification for it. The general rule really
should be to just WARN and hope the driver survives for long enough.

v2: Make the WARN a bit more useful per Chris' suggestion.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>