When turning off the engines, and the pmu sampling, clear the previous
value as the current measurement should be 0.

v2: Use a for-loop
v3:
 * Move clearing to timer self-dis-arm to avoid race with parking.
 * Clear frequency samples as well.
v4:
 * Init frequency to idle_freq. (Chris Wilson)
Signed-off-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v3)
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123102654.29296-1-tvrtko.ursulin@linux.intel.com