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>

drivers/gpu/drm/i915/intel_lrc.c

@@ -742,6 +742,7 @@ static int execlists_context_pin(struct intel_engine_cs *engine,
 
 	if (ce->pin_count++)
 		return 0;
+	GEM_BUG_ON(!ce->pin_count); /* no overflow please! */
 
 	if (!ce->state) {
 		ret = execlists_context_deferred_alloc(ctx, engine);

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1445,6 +1445,7 @@ static int intel_ring_context_pin(struct intel_engine_cs *engine,
 
 	if (ce->pin_count++)
 		return 0;
+	GEM_BUG_ON(!ce->pin_count); /* no overflow please! */
 
 	if (ce->state) {
 		ret = context_pin(ctx);