intel_th: gth: Factor out trace start/stop

The trace enable/disable functions of the GTH include the code that starts
and stops trace flom from the sources. This start/stop functionality will
also be used in the window switch trigger sequence.

Factor out start/stop code from the larger trace enable/disable code in
preparation for the window switch sequence.

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