[RFC] Common DRM execution context v2

[Christian König]

Hello everyone,

compared to the earlier version this has seen quite a bit of additional
testing and can now handle both amdgpu as well as radeon without any
performance drop.

QXL is converted over as well and then the remaining ttm_execbuf_util
implementation moved into VMWGFX which is the only remaining user of
this.

Please review and comment,
Christian.