drm/i915: Don't call sg_free_table() if sg_alloc_table() fails

One needs to call __sg_free_table() if __sg_alloc_table() fails, but
sg_alloc_table() does that for us already.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewd-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>