drm/i915: Do not rely on for loop caching the mask

for_each_engine_masked caches the engine mask but what does the caller
know.

Cache it explicitly for clarity and while at it correct the type to match.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725125056.11942-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/i915_active.c

@@ -378,12 +378,12 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
 					    struct intel_engine_cs *engine)
 {
 	struct drm_i915_private *i915 = engine->i915;
+	intel_engine_mask_t tmp, mask = engine->mask;
 	struct llist_node *pos, *next;
-	unsigned long tmp;
 	int err;
 
-	GEM_BUG_ON(!engine->mask);
-	for_each_engine_masked(engine, i915, engine->mask, tmp) {
+	GEM_BUG_ON(!mask);
+	for_each_engine_masked(engine, i915, mask, tmp) {
 		struct intel_context *kctx = engine->kernel_context;
 		struct active_node *node;