drm/sysfs: Do not drop device reference twice

device_unregister() already drops its reference to the struct device, so
explicitly calling put_device() before device_unregister() can cause the
device to have been freed before it can be unregistered.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>