drm/i915: Wait for the clocks to stabilise before updating PLLs

When initialising the PLL registers we may have to clear existing state
from the BIOS - that is the PLL may already be enabled. So we need to
disable it, wait for the clocks to settle and then rewrite it.

The issue came to light when Ben tested

commit 88ca4bb7974277793e602d88739d4e8f56b89e64
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Apr 20 17:11:53 2012 +0100

    drm/i915: manage PCH PLLs separately from pipes

and found that booting into a VGA monitor was no longer working. Closer
inspection suggests that it was a pre-existing bug now being hit by the
rearranged code. Perhaps Ben was not even the first person to stumble
upon this bug, https://bugs.freedesktop.org/show_bug.cgi?id=37029

.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reported-and-Tested-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>