Gang submit v2

[Christian König]

Hi guys,

secound round for this patch set. I've fixed the minor comments and bugs
Andrey and Yogesh came up with and rebased everything.

The base is not the current amd-staging-drm-next branch, but rather Alex
rebased version since this depends on the dma-resv usage rework. So
don't bother trying to apply that anywhere.

The patches are also available here https://gitlab.freedesktop.org/ckoenig/linux-drm.git
on branch amd-gang-submit.

Please review and comment,
Christian.