drm/i915: make runtime PM swizzling/ring_freq init platform independent

drm/i915: make runtime PM swizzling/ring_freq init platform independent

We need to re-init sizzling on all platforms so move it to the
platform independent runtime resume callback. The ring frequency reinit
is also needed everywhere except on VLV, but gen6_update_ring_freq()
will be a noop on VLV, so we can move this function too to platform
independent code.

Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>