drm/i915: Assert that the context pin_counts do not overflow

drm/i915: Assert that the context pin_counts do not overflow

This should be impossible, but let's assert that we do not pin a context
4 billion times before retiring!

v2: Fix the assertion -- the patch had just one job to do!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170316171628.3228-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>