drm/i915: Add FIXME for bdw semaphore detection in hancheck

Currently not an issue since we don't emit sempahores, but better
not forget about those.

As a little prep work extract the ipehr decoding for cleaner control
flow. And apply a bit of polish.

Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>