Common DRM execution context v3

[Christian König]

Hi guys,

thrid round for those patches. They have been in my queue for nearly a
year now because I couldn't find much time to push into this.

Danilo wants to use this for his GPU VAs tracker work and Arun needs it
for hist secure semaphore work, so we should probably get it reviewed
now.

Compared to the last version I've fixed one memory leak found by Danilo
and removed the support for duplicate tracking. Only radeon really needs
that and we can trivially handle it differently there.

Please review and/or comment,
Christian.