drm/i915: always set FDI composite sync bit

It's needed for 3 pipe support as well as just regular functionality
(e.g. DisplayPort).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Adam Jackson <ajax@redhat.com>
Tested-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>