drm/i915: Prevent divide-by-zero in debugfs/i915_rps_boost_info

Either by chance, or by misread, the current evaluation interval may be
zero. If that is the case, don't divide by it!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170218112708.24504-1-chris@chris-wilson.co.uk


Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>