drm/i915: add i9xx pfit pipe asserts

We can only enable the pfit if the pipe is disabled. Ensure that this
is obeyed with a neat assert.

Also check whether the pfit is off before enabling it - if not we've
lost track of things somewhere since the pfit is only ever used by the
lvds output.

v2: Fix spell fail in the commit message pointed out by Ville&Jani.

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