[rbp/omxplayer] Avoid loss of volume messages

There seems to be a race condition where the volume request arrives as the omx audio pipeline is being created.
Restructure the volume messages to apply the initial volume as pipeline is being created, and add missing locks.
This also reduces the differences between OMXPlayer and DVDPlayer
Merge pull request #3110 from popcornmix/volumeloss

[rbp/omxplayer] Avoid loss of volume messages