[PATCH] drm/i915: Fix an error checking test

[Chris Wilson]

Quoting Christophe JAILLET (2017-06-27 06:38:54)
> 'dma_buf_vmap' returns NULL on error, not an error pointer.
> 
> Fixes: 6cca22ede8a4 ("drm/i915: Add some mock tests for dmabuf interop")
> Signed-off-by: Christophe JAILLET <christophe.jaillet at wanadoo.fr>

Thanks for the fix, reviewed and pushed.
-Chris