drm: add control node checks missing from kms merge

This line that checks the DRM_CONTROL_ALLOW flag was missed from the KMS
merge.  Re-add the check on the IOCTL, as this is currently the only use of
this flag.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Dave Airlie <airlied@redhat.com>