drm/i915: Correct error handling for i915_gem_object_create_from_data()

i915_gem_object_create_from_data() always returns an error pointer on
failure, there is no need to check against NULL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317205317.7885-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>