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 Job ID Name Coverage
  Manifest
passed #43890
manifest

00:00:20

 
  Build
passed #43891
build fedora x86_64

00:01:17