drm/i915: Enable the PCH PLL for all generations after link training

Hidden away within one chipset specific path was the necessary logic to
turn on the PLL. This needs to be done everywhere in order for us to
drive any display! As such as soon as we tested on a non-CougarPoint
chipset, we failed to bring up any DisplayPorts and generated a nice set
of assertion failures in the process. At least one part of our logic is
working, the part that assumes that we have no idea what we are doing.

Reported-by:  <guang.a.yang@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=49712


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>