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

drivers/gpu/drm/i915/gem/selftests/i915_gem_coherency.c

@@ -327,6 +327,7 @@ static int igt_gem_coherency(void *arg)
 	ctx.engine = random_engine(i915, &prng);
 	GEM_BUG_ON(!ctx.engine);
 	pr_info("%s: using %s\n", __func__, ctx.engine->name);
+	intel_engine_pm_get(ctx.engine);
 
 	for (over = igt_coherency_mode; over->name; over++) {
 		if (!over->set)
@@ -405,6 +406,7 @@ static int igt_gem_coherency(void *arg)
 		}
 	}
 free:
+	intel_engine_pm_put(ctx.engine);
 	kfree(offsets);
 	return err;