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