drm/i915: Support for RR switching on VLV

Definition of VLV RR switch bit and corresponding toggling in
set_drrs function.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>