tvservice: Add callback unregister with data pointer check

In some circumstances an application may register multiple callback instances
using the same function pointer but differerent data context. To allow proper
unregister of these callbacks add a new method which checks for function
pointer and data pointer before removal.

Signed-off-by: Julian Scheel <julian@jusst.de>
tvservice: Increase max callback count to 5

A maximum callback count of 2 might be too low for some usecases.

Signed-off-by: Julian Scheel <julian@jusst.de>
Merge pull request #162 from julianscheel/master

tvservice: Enhance callback mechanism