drm/i915: WARN if module opt sanitization goes out of order

Depending upon one module option to be sanitized (through USES_PPGTT)
for the other is a bit too fragile for my taste. At least WARN about
this.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -43,6 +43,8 @@
 
 int intel_sanitize_enable_execlists(struct drm_device *dev, int enable_execlists)
 {
+	WARN_ON(i915.enable_ppgtt == -1);
+
 	if (enable_execlists == 0)
 		return 0;