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

Higher RC6 residency is observed using timeout mode
instead of EI mode. It's Recommended to use TO Method for RC6.

v2: Add comment about timeout threshold. (Tom)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>