drm/i915: Don't leave old junk in ilk active watermarks on readout

When we read out the watermark state from the hardware we're supposed to
transfer that into the active watermarks, but currently we fail to any
part of the active watermarks that isn't explicitly written. Let's clear
it all upfront.

Looks like this has been like this since the beginning, when I added the
readout. No idea why I didn't clear it up.

Cc: Matt Roper <matthew.d.roper@intel.com>
Fixes: 243e6a44b9ca ("drm/i915: Init HSW watermark tracking in intel_modeset_setup_hw_state()")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463151318-14719-2-git-send-email-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -4093,6 +4093,8 @@ static void ilk_pipe_wm_get_hw_state(struct drm_crtc *crtc)
 	if (IS_HASWELL(dev) || IS_BROADWELL(dev))
 		hw->wm_linetime[pipe] = I915_READ(PIPE_WM_LINETIME(pipe));
 
+	memset(active, 0, sizeof(*active));
+
 	active->pipe_enabled = intel_crtc->active;
 
 	if (active->pipe_enabled) {