drm/i915/error: Check the potential ctx obj's vm

drm/i915/error: Check the potential ctx obj's vm

The bound list is global (all objects which back the VMAs are stored
here). Recently the BUG() in the offset lookup was demoted to a WARN,
but the fault actually lies in the caller, here.

This bug has existed since the initial introduction of PPGTT (however,
it was fixed in unmerged patches to fix up the error state).

Note: The reason for the BUG_ON to WARN_ON demotion was _not_ to
duct-tape over this bug here but another but triggerable without
ppgtt. See the commit for details:

commit f25748ea7342bcc44866befcbf0222cd1d89d851
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Jun 17 22:34:38 2014 +0200

    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>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Clarfy that the WARN_ON (former BUG_ON) in ggtt_offset caught
more than just this bug fixed in this patch here.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>