drm/i915: set watermarks for third pipe on IVB

The watermark reg for the third pipe is in an unusual offset; add
support for it and set watermarks for 3 pipe configs.

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