drm/i915: Return error from i915_gem_object_get_fence_reg() when failing.

Previously, the caller would continue along without knowing that the
function failed, resulting in potential mis-rendering.  Right now vm_fault
just returns SIGBUS in that case, and we may need to disable signal handling
to avoid that happening.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>