drm/i915/selftests: Keep engine awake during live_coherency

Keep the engine awake and so avoid frequent cycling in and out of
powersaving mode to eliminate the unnecessary overhead and speed up the
testing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129222702.1456292-1-chris@chris-wilson.co.uk