drm/i915: disable contexts on old HW

This got dropped as a result of the last round of comments. I didn't
test it on unsupported HW (which this is likely the case).

Note that this prevents hw context from blowing up on any pre-gen6 hw.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51142
[danvet: Added note and buglink.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -243,8 +243,10 @@ void i915_gem_context_init(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	uint32_t ctx_size;
 
-	if (!HAS_HW_CONTEXTS(dev))
+	if (!HAS_HW_CONTEXTS(dev)) {
+		dev_priv->hw_contexts_disabled = true;
 		return;
+	}
 
 	/* If called from reset, or thaw... we've been here already */
 	if (dev_priv->hw_contexts_disabled ||