drm/i915: Try enabling RC6 by default (again)

Jesse Barnes and I found a couple of issues where incorrect mode
setting would cause problems with RC6 enabled. We're hopeful that
fixing those will resolve the outstanding issues with a few machines
that had trouble before 3.0 with rc6.

Cc: Pekka Enberg <penberg@kernel.org>
Cc: Francesco Allertsen <fallertsen@gmail.com>
Cc: Ted Phelps <phelps@gnusto.com>
Cc: Gu Rui <chaos.proton@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38567
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=38332


Signed-off-by: Keith Packard <keithp@keithp.com>