drm/i915: handle 3rd pipe

We don't need to check 3rd pipe specifically, as it shares PLL with some
other one.

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