intel_th: msu: Factor out pipeline draining

The code that waits for the pipeline empty condition of the MSU is
currently called in the path that disables the trace. We will also
need this in the window switch trigger sequence. Therefore, factor
out this code and make it accessible to the GTH device.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>