drm/i915: Limit number of reads to stabilize rc6 counter reads

drm/i915: Limit number of reads to stabilize rc6 counter reads

We have only 8bits of precise timestamps in which to complete our
upper/load reads, along with the switch between precision. This is not
always enough time to read the upper counter twice within the same time
slice, leading to hard lockups. Limit the number of times to prevent
an inifite loop (my fault for assuming we would have no trouble doing
the write + reads fast enough).

Fixes: a89dd1365b5d ("drm/i915: Extend vlv/chv residency resolution")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100377
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324165418.7455-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>