drm/i915: Make sure LP1+ watermarks levels are preserved when going from 1 to 2 pipes

Once again ILK is unhappy if we clear out the LP1+ watermark levels
outright, and instead we must disable the levels we don't want while
still leaving the actual programmed watermark levels intact.

Fixes underruns on the already enabled pipe when programming watermarks
while enabling the second pipe.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Matt Roper <matthew.d.roper@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93787
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459536799-18109-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -2483,7 +2483,7 @@ static void ilk_wm_merge(struct drm_device *dev,
 	/* ILK/SNB/IVB: LP1+ watermarks only w/ single pipe */
 	if ((INTEL_INFO(dev)->gen <= 6 || IS_IVYBRIDGE(dev)) &&
 	    config->num_pipes_active > 1)
-		return;
+		last_enabled_level = 0;
 
 	/* ILK: FBC WM must be disabled always */
 	merged->fbc_wm_enabled = INTEL_INFO(dev)->gen >= 6;