drm/i915: Increase max fence pitch limit to 256KB on IVB+

BSpec contains several scattered notes which state that the maximum
fence stride was increased to 256KB on IVB.

Testing on real hardware agrees.

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