vaapiencode: don't start src pad task in set_format

Otherwise the task may be restarted during shutdown. Start the task in
gst_vaapiencode_handle_frame() instead.
2 jobs for task-start in 1 minute and 37 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Manifest
passed manifest #43890

00:00:20

 
  Build
passed build fedora x86_64 #43891

00:01:17