defconfig: 8974: disable RTB logging in perf defconfig

RTB logging adds latency for the register access. This is causing
hindrance to the performance measurement. Hence disable RTB logging
in perf defconfig.

Change-Id: I8e3eb552a667e7958621c91b6b779b9fce24b350
Signed-off-by: Ping Li <quicpingli@codeaurora.org>