drm/i915: Remove unused dot_limit from VLV PLL limits

We don't use .dot_limit for anything on VLV, so don't populate it.

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