drm/i915: Warn when DPIO read returns 0xffffffff

DPIO reads from groups/broadcast register offsets for PCS and
TX return all 1's. If that result gets used for something
we'll probably end up doing something wrong. So warn when that
happens.

FIXME there might be some registers where all 1's is a valid value,
so ideally we should check the register offset instead...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: copypaste the FIXME comment into the code.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>