drm/i915: Convert intel_rc6_residency_us to ns

Will be used for exposing the PMU counters.

v2:
 * Move intel_runtime_pm_get/put to the callers. (Chris Wilson)
 * Restore full unit conversion precision.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171121181852.16128-8-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_sysfs.c

@@ -42,8 +42,13 @@ static inline struct drm_i915_private *kdev_minor_to_i915(struct device *kdev)
 static u32 calc_residency(struct drm_i915_private *dev_priv,
 			  i915_reg_t reg)
 {
-	return DIV_ROUND_CLOSEST_ULL(intel_rc6_residency_us(dev_priv, reg),
-				     1000);
+	u64 res;
+
+	intel_runtime_pm_get(dev_priv);
+	res = intel_rc6_residency_us(dev_priv, reg);
+	intel_runtime_pm_put(dev_priv);
+
+	return DIV_ROUND_CLOSEST_ULL(res, 1000);
 }
 
 static ssize_t