drm/i915/bdw: Use timeout mode for RC6 on bdw

Higher RC6 residency is observed using timeout mode
instead of EI mode.  This applies to Broadwell only.
The difference is particularly noticeable with video
playback.

Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>