'dma_buf_vmap' returns NULL on error, not an error pointer.

Fixes: 6cca22ed ("drm/i915: Add some mock tests for dmabuf interop")
Signed-off-by: NChristophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627053854.21152-1-christophe.jaillet@wanadoo.frReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>