drm mode: Return -errno on drmIoctl() failure

The high layers expect to receive a status code on error (on the
pessimistic assumption that the errno value will have been overwritten
by the time the failure is propagated all the way up), so convert
xf86drmMode.c to return -errno on an ioctl error and be consistent with
the rest of the libdrm API.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>