drm/i915: Assert that the transcoder is indeed off before modifying it

Inspired by a recent regression that seems to confuse pch transcoder
state, let's be a bit more paranoid.

References: https://bugs.freedesktop.org/show_bug.cgi?id=49712
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Pimped commit message.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2782,6 +2782,8 @@ static void ironlake_pch_enable(struct drm_crtc *crtc)
 	int pipe = intel_crtc->pipe;
 	u32 reg, temp;
 
+	assert_transcoder_disabled(dev_priv, pipe);
+
 	/* For PCH output, training FDI link */
 	dev_priv->display.fdi_link_train(crtc);