drm/i915: Don't do PM5/DDR DVFS with multiple pipes

Enabling PM5/DDR DVFS with multiple active pipes isn't a validated
configuration. It does seem to work most of the time at least, but
there is clearly an additional risk of underruns, so let's not play
with fire.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NClint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -1327,6 +1327,9 @@ static void vlv_merge_wm(struct drm_device *dev,
 	if (num_active_crtcs != 1)
 		wm->cxsr = false;
 
+	if (num_active_crtcs > 1)
+		wm->level = VLV_WM_LEVEL_PM2;
+
 	for_each_intel_crtc(dev, crtc) {
 		struct vlv_wm_state *wm_state = &crtc->wm_state;
 		enum pipe pipe = crtc->pipe;