drm/i915: fix PCH PLL assertion check for 3 pipes

Add a couple of checks now that we're using the 3rd transcoder:
  1) make sure the transcoder PLL enable bit is set for the transcoder
     in question
  2) when checking actual PLL enable, use the selected PLL number rather
     than the transcoder number (they could be different now)

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>