drm/i915: Fix possible uninitialized variable in parallel extension

'prev_engine' was declared inside the output loop and checked in the
inner after at least 1 pass of either loop. The variable should be
declared outside both loops as it needs to be persistent across the
entire loop structure.

Fixes: e5e32171 ("drm/i915/guc: Connect UAPI to GuC multi-lrc interface")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211219001909.24348-1-matthew.brost@intel.com
(cherry picked from commit cbffbac9)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -564,6 +564,7 @@ set_proto_ctx_engines_parallel_submit(struct i915_user_extension __user *base,
 		container_of_user(base, typeof(*ext), base);
 	const struct set_proto_ctx_engines *set = data;
 	struct drm_i915_private *i915 = set->i915;
+	struct i915_engine_class_instance prev_engine;
 	u64 flags;
 	int err = 0, n, i, j;
 	u16 slot, width, num_siblings;
@@ -629,7 +630,6 @@ set_proto_ctx_engines_parallel_submit(struct i915_user_extension __user *base,
 	/* Create contexts / engines */
 	for (i = 0; i < width; ++i) {
 		intel_engine_mask_t current_mask = 0;
-		struct i915_engine_class_instance prev_engine;
 
 		for (j = 0; j < num_siblings; ++j) {
 			struct i915_engine_class_instance ci;