drm/i915: Add IVB DDB partitioning control

On IVB the display data buffer partitioning control lives in the
DISP_ARB_CTL2 register. Add the relevant defines/code for it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>