drm/i915: set proper DPIO post divider for VGA on VLV v4

Supposedly we should use the DAC divider for <300MHz pixel clocks, but as
that doesn't actually work as well as the high freq divider here in
practice, just use the high freq divider all the time.

v2: remove unconditional write (Jesse)
    check for pixel rate properly (Jesse)
v3: give up, the DAC divider apparently doesn't work, and low res modes
    work ok (Jesse)
    remove debug msg (Jesse)
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -4468,10 +4468,13 @@ static void vlv_update_pll(struct intel_crtc *crtc)
 	mdiv |= ((bestp1 << DPIO_P1_SHIFT) | (bestp2 << DPIO_P2_SHIFT));
 	mdiv |= ((bestn << DPIO_N_SHIFT));
 	mdiv |= (1 << DPIO_K_SHIFT);
-	if (intel_pipe_has_type(&crtc->base, INTEL_OUTPUT_HDMI) ||
-	    intel_pipe_has_type(&crtc->base, INTEL_OUTPUT_EDP) ||
-	    intel_pipe_has_type(&crtc->base, INTEL_OUTPUT_DISPLAYPORT))
-		mdiv |= (DPIO_POST_DIV_HDMIDP << DPIO_POST_DIV_SHIFT);
+
+	/*
+	 * Post divider depends on pixel clock rate, DAC vs digital (and LVDS,
+	 * but we don't support that).
+	 * Note: don't use the DAC post divider as it seems unstable.
+	 */
+	mdiv |= (DPIO_POST_DIV_HDMIDP << DPIO_POST_DIV_SHIFT);
 	intel_dpio_write(dev_priv, DPIO_DIV(pipe), mdiv);
 
 	mdiv |= DPIO_ENABLE_CALIBRATION;