msm: rtb: Log the context id in the rtb

Store the context id in the register trace buffer.
The process id can be derived from the context id.
This gives a general idea about what process was last
running when the RTB stopped.

Change-Id: I34db8135b78b232190044f771e61639065d73318
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>