drm/amdgpu: add gang submit backend v2

Allows submitting jobs as gang which needs to run on multiple
engines at the same time.

Basic idea is that we have a global gang submit fence representing when the
gang leader is finally pushed to run on the hardware last.

Jobs submitted as gang are never re-submitted in case of a GPU reset since this
won't work and will just deadlock the hardware immediately again.

v2: fix logic inversion, improve documentation, fix rcu

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>