drm/i915: enable rc6 on ilk again

I have the faint hope that the total absence of any locking for the
rps code wasn't too good an idea and could very well have caused some
rc6 related regressions.

Unfortunately we've never managed to reproduce these issues on any of
our own machines, so the only way to go about this is to enable it and
see what happens.

While at it, kill some stale comments and improve the logging.

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>