drm: check return values in drm_version

In drm_version, actually check the results from function calls so that
we're not potentially passing garbage back to userspace.

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>