sched/debug: use symbolic names for task state constants

include/trace/events/sched.h includes <linux/sched.h> (via
<linux/sched/numa_balancing.h>) and so knows about the TASK_* constants
used to interpret .prev_state. So instead of duplicating the magic
numbers make use of the defined macros to ease understanding the
mapping from state bits to letters which isn't completely intuitive for
an outsider.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kernel@pengutronix.de
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180905093636.24068-1-u.kleine-koenig@pengutronix.de