drm/i915: prepare bind_to_vm for preallocated vma

In the new execbuf code we want to track buffers using the vmas even
before they're all properly mapped. Which means that bind_to_vm needs
to deal with buffers which have preallocated vmas which aren't yet
bound.

This patch implements this prep work and adjusts our WARN/BUG checks.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Split out from Ben's big execbuf patch. Also move one BUG
back to its original place to deflate the diff a notch.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>