CHROMIUM: enable i915 power-saving render C-state 6 by default.

CHROMIUM: enable i915 power-saving render C-state 6 by default.

BUG=chrome-os-partner:6768
TEST=manual,
- boot kernel
1. cat /sys/module/i915/parameters/i915_enable_rc6
  - should equal 1
2. start powertop, goto 'Idle Stats' tab
  - make sure that the Package c-states (C6, C7) are entered at some
    non-trivial percentage when system is idle

Signed-off-by: Todd Broch <tbroch@chromium.org>
Change-Id: Ib557a14e3cf20b55a6670808e681ef21b1e0ed1b
Reviewed-on: https://gerrit.chromium.org/gerrit/12280
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>