coresight-tmc-etr: Fix deadlock issue while switching mode

In ETR mode switch stress test, there will be a deadlock issue if
fail to enable ETM source. Since switch mode function will hold
mem_lock and call tmc_disable_etr_sink if ETM failed to be enabled.
Meanwhile, tmc_disable_etr_sink also need to hold mem_lock. Release
mem_lock before calling coresight_enable_all_source_link to fix this
deadlock issue.

Change-Id: I0e9040e79e8ef5278f7b279ce2f7265147ef2b74
Signed-off-by: Tao Zhang <taozha@codeaurora.org>