coresight: tmc: leverage etr disable/enable when switching mode

Disable ETR first and enable it again with new mode to switch
out_mode. In this way, we can leverage etr_disable and etr_enable
function and avoid doing same thing.

Change-Id: I5dcae31b657568c751e9b4a209cc7a85d13f83f0
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>