msm: qdss: introduce mutex locking for funnel_enable/disable

In preparation for other qdss source drivers to use the qdss APIs
ensure that funnel_enable and funnel_disable are properly protected
by locks.

Also default the funnel priority for all ports to zero so that all
sources get equal priority.

This is required since going forward there will be more than one
user of the qdss driver.

Change-Id: I8fec21f88e5a206c019d66a50570e5fc91ba6044
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>