drm/i915: Add some extra guards in evict_vm

v2: Use WARN_ONs (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>