omxvideo{enc,dec}: stop calling shutdown() in change_state

omxvideo{enc,dec}: stop calling shutdown() in change_state

This is no longer needed since we implemented close() vfuncs as the
encoder/decoder base class already take care of calling close() (which
is calling shutdown()) in its own change_state implementation.

We also move the shut down of the component from PAUSED_TO_READY to READY_TO_NULL.
By doing so upstream will have already deactivated the pool from the
encoder and so won't be preventing the OMX state change as the buffers
will all be released.

https://bugzilla.gnome.org/show_bug.cgi?id=796918