drm/i915: Prevent a machine hang by checking crtc->active before loading lut

Before loading the lut (gamma), check the active state of intel_crtc,
otherwise at least on gen2 hang ensue.

This is reproducible in Xorg via:
  xset dpms force off
then
  xgamma -rgamma 2.0 # freeze.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44505
Signed-off-by: Alban Browaeys <prahal@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_display.c

@@ -6184,7 +6184,7 @@ void intel_crtc_load_lut(struct drm_crtc *crtc)
 	int i;
 
 	/* The clocks have to be on to load the palette. */
-	if (!crtc->enabled)
+	if (!crtc->enabled || !intel_crtc->active)
 		return;
 
 	/* use legacy palette for Ironlake */