drm/i915: grab force_wake when restoring LCPLL

If LCPLL is disabled, there's a chance we might be in package C8 state
or deeper, and we'll get a hard hang when restoring LCPLL (also, a red
led lights up on my motherboard). So grab the force_wake, which will
get us out of RC6 and, as a consequence, out of PC8+ (since we need
RC6 to get into PC8+).

Note: Discussions with hw designers are still ongoing what exactly
goes boom here. But I think we can go ahead and just merge this little
hack for now until it's clear what we actually need.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Add small note about the current state of the discussion
around this hack.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -6033,6 +6033,10 @@ void hsw_restore_lcpll(struct drm_i915_private *dev_priv)
 		    LCPLL_POWER_DOWN_ALLOW)) == LCPLL_PLL_LOCK)
 		return;
 
+	/* Make sure we're not on PC8 state before disabling PC8, otherwise
+	 * we'll hang the machine! */
+	dev_priv->uncore.funcs.force_wake_get(dev_priv);
+
 	if (val & LCPLL_POWER_DOWN_ALLOW) {
 		val &= ~LCPLL_POWER_DOWN_ALLOW;
 		I915_WRITE(LCPLL_CTL, val);
@@ -6060,6 +6064,8 @@ void hsw_restore_lcpll(struct drm_i915_private *dev_priv)
 					LCPLL_CD_SOURCE_FCLK_DONE) == 0, 1))
 			DRM_ERROR("Switching back to LCPLL failed\n");
 	}
+
+	dev_priv->uncore.funcs.force_wake_put(dev_priv);
 }
 
 static void haswell_modeset_global_resources(struct drm_device *dev)