drm/i915: Fix an error checking test

drm/i915: Fix an error checking test

'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@wanadoo.fr>
Link: http://patchwork.freedesktop.org/patch/msgid/20170627053854.21152-1-christophe.jaillet@wanadoo.fr
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit 7c3f5317b8c2828ab10e8cf87c8ab5232d1966d0)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>