drm/i915/lvds: Ensure panel is unlocked for Ironlake or the panel fitter

Commit 77d07fd9 introduced a regression
where by not waiting for the panel to be turned off, left the panel and
PLL registers locked across the modeset. Thus the panel remaining blank.

As pointed out by Daniel Vetter, when testing LVDS it helps to open the
laptop and look at the actual panel you are purporting to test.

A second issue with the patch was that in order to modify the panel
fitter before gen5, the pipe and the panel must have be completely
powered down. So we wait.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>