coresight: etm4x: Add lock for reading virtual context ID comparator

Updates to the values and the index are protected via the spinlock.
Ensure we use the same lock to read the value safely.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20220204152403.71775-2-leo.yan@linaro.org