coresight-tmc: Avoid reading TMC buffer before enabling

Add sticky enable flag to avoid reading TMC buffer before
enabling TMC at least once.

Change-Id: Id1c04101d65923c266242370f725ea611b0ac846
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>