drm/i915: Satisfy smatch that a loop has at least one iteration

Smatch worries that the engine->mask may be 0 leading to the loop being
shortcircuited leaving the next pointer unset,

drivers/gpu/drm/i915/i915_active.c:667 i915_active_acquire_preallocate_barrier() error: uninitialized symbol 'next'.

Assert that mask is not 0 and smatch can then verify that next must be
initialised before use.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117110603.2982286-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_active.c

@@ -621,6 +621,7 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
 	 * We can then use the preallocated nodes in
 	 * i915_active_acquire_barrier()
 	 */
+	GEM_BUG_ON(!mask);
 	for_each_engine_masked(engine, gt, mask, tmp) {
 		u64 idx = engine->kernel_context->timeline->fence_context;
 		struct active_node *node;