lib: add igt_vlog to print varargs

... and put it to immediate use in igt_display_log.

To make this all add up also drop the return value of igt_display_log,
no one really cared anyway.

Aside: I've noticed that every time another subtest runs (at least
with kms_pipe_crc_basic) the log indent level moves one up ...

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

lib/igt_core.h

@@ -389,6 +389,7 @@ enum igt_log_level {
 };
 __attribute__((format(printf, 2, 3)))
 void igt_log(enum igt_log_level level, const char *format, ...);
+void igt_vlog(enum igt_log_level level, const char *format, va_list args);
 
 /**
  * igt_debug: