drm/i915: Try to print INSTDONE bits for all slice/subslice

v2: (Imre)
- Access only subslices that are known to exist.
- Reset explicitly the MCR selector to slice/sub-slice ID 0 after the
  readout.
- Use the subslice INSTDONE bits for the hangcheck/subunits-stuck
  detection too.
- Take the uncore lock for the MCR-select/subslice-readout sequence.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1474379673-28326-2-git-send-email-imre.deak@intel.com