drm/i915: fix ACTHD for gen <= 3

This was mixed up in the following patch:

commit a6c45cf013a57e32ddae43dd4ac911eb4a3919fd
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Sep 17 00:32:17 2010 +0100

    drm/i915: INTEL_INFO->gen supercedes i8xx, i9xx, i965g

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

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -131,7 +131,7 @@ static unsigned int render_ring_get_active_head(struct drm_device *dev,
 						struct intel_ring_buffer *ring)
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
-	u32 acthd_reg = INTEL_INFO(dev)->gen ? ACTHD_I965 : ACTHD;
+	u32 acthd_reg = INTEL_INFO(dev)->gen >= 4 ? ACTHD_I965 : ACTHD;
 
 	return I915_READ(acthd_reg);
 }