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>

drivers/gpu/drm/i915/intel_lvds.c

@@ -159,6 +159,9 @@ static void intel_pre_enable_lvds(struct intel_encoder *encoder)
 	if (HAS_PCH_SPLIT(dev) || !enc->pfit_control)
 		return;
 
+	WARN_ON(I915_READ(PFIT_CONTROL) & PFIT_ENABLE);
+	assert_pipe_disabled(dev_priv, to_intel_crtc(encoder->base.crtc)->pipe);
+
 	/*
 	 * Enable automatic panel scaling so that non-native modes
 	 * fill the screen.  The panel fitter should only be