drm/i915/bdw: Add comment about gen8 HWS PGA

This confused me some many times that I think it is appropriate to add a
small comment to instruct the reader of the code that it is indeed doing
what it is supposed to do.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>