drm/i915: Do not set "Enable Panel Fitter" on SNB pageflips

Not only do the pageflip work without it at non-native modes (i.e. with
the panel fitter enabled), it also causes normal (non-pageflipped)
modesets to fail.

Reported-by: Adam Jackson <ajax@redhat.com>
Tested-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Wanted-by-for-fixes: Dave Airlie <airlied@gmail.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -7468,7 +7468,13 @@ static int intel_gen6_queue_flip(struct drm_device *dev,
 	OUT_RING(fb->pitches[0] | obj->tiling_mode);
 	OUT_RING(obj->gtt_offset);
 
-	pf = I915_READ(PF_CTL(intel_crtc->pipe)) & PF_ENABLE;
+	/* Contrary to the suggestions in the documentation,
+	 * "Enable Panel Fitter" does not seem to be required when page
+	 * flipping with a non-native mode, and worse causes a normal
+	 * modeset to fail.
+	 * pf = I915_READ(PF_CTL(intel_crtc->pipe)) & PF_ENABLE;
+	 */
+	pf = 0;
 	pipesrc = I915_READ(PIPESRC(intel_crtc->pipe)) & 0x0fff0fff;
 	OUT_RING(pf | pipesrc);
 	ADVANCE_LP_RING();