drm/i915: Sanitize shared dpll state

There seems to be no limit to the amount of gunk the firmware can
leave behind. Some platforms leave pch dplls on which are not in
active use at all. The example in the bug report is a Apple Macbook
Pro.

Note that this escape scrunity of the hw state checker until we've
tried to use this enabled, but unused pll since we did only check for
the inverse case of a in-used, but disabled pll.

v2: Add a WARN in the pll state checker which would have caught this
case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=66952


Reported-and-tested-by: shui yangwei <yangweix.shui@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>