drm/i915: Enable Resource Streamer state save/restore on MI_SET_CONTEXT

Also clarify comments on context size that the extra state for
Resource Streamer is included.

v2: Don't remove the extended save/restore enabled for older
    platforms. (Ville)
    Use new MI_SET_CONTEXT defines for HSW RS save/restore state
    instead of extended save/restore. (Daniel)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>