drm/i915: Show vma allocator stack when in doubt

At the moment, gem_exec_gttfill fails with a sporadic EBUSY due to us
wanting to unbind a pinned batch. Let's dump who first bound that vma to
see if that helps us identify who still unexpectedly has it pinned.

v2: We cannot allocate inside the printer (as it may be on an fs-reclaim
path), so hope for the best and build the string on the stack
v3: stack depth of 16 routinely overflows a 512 character string, limit
it to 12 to avoid unsightly truncation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180628132206.8329-1-chris@chris-wilson.co.uk