drm/i915/guc: Don't go bang in GuC log if no GuC

If the GuC has failed to load for any reason and then the user pokes
the debugfs GuC log interface, a BUG and/or null pointer deref can
occur. Don't let that happen.

Signed-off-by: John Harrison <[e-mail]>
Reviewed-by: Lucas De Marchi <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gt/uc/intel_guc_log_debugfs.c

···
 {
 	struct intel_guc_log *log = data;
 
-	if (!intel_guc_is_used(log_to_guc(log)))
+	if (!log->vma)
 		return -ENODEV;
 
 	*val = intel_guc_log_get_level(log);
···
 {
 	struct intel_guc_log *log = data;
 
-	if (!intel_guc_is_used(log_to_guc(log)))
+	if (!log->vma)
 		return -ENODEV;
 
 	return intel_guc_log_set_level(log, val);