drm/i915: Show render P state thresholds in sysfs

This is useful for userspace utilities which wish to use the previous
interface, specifically for micromanaging the increase/decrease steps by
setting min == max.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>