drm/i915: check for multiple write domains in pin_and_relocate

The assumption that an object has only ever one write domain is deeply
threaded into gem (it's even encoded the the singular of the variable
name). Don't let userspace screw us over.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>