drm/i915: add S PLL control

This PLL control can drive DDI ports at desired frequencies for
DisplayPort and FDI connections.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>