arch/x86_64: Fix up_timer_start

Fix according to up_alarm_start.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

arch/x86_64/src/intel64/intel64_tsc_tickless.c

@@ -303,6 +303,8 @@ int up_timer_start(const struct timespec *ts)
 
   up_tmr_sync_up();
 
+  up_unmask_tmr();
+
   ticks = up_ts2tick(ts) + rdtscp();
 
   g_timer_active = 1;
@@ -311,8 +313,6 @@ int up_timer_start(const struct timespec *ts)
 
   g_goal_time = ticks;
 
-  up_unmask_tmr();
-
   up_tmr_sync_down();
   return OK;
 }