drm/amdgpu: completely rework eviction fence handling v2

drm/amdgpu: completely rework eviction fence handling v2

Well that was broken on multiple levels.

First of all a lot of checks were placed at incorrect locations, especially if
the resume worker should run or not.

Then a bunch of code was just mid-layering because of incorrect assignment who
should do what.

And finally comments explaining what happens instead of why.

Just re-write it from scratch, that should at least fix some of the hangs we
are seeing.

Use RCU for the eviction fence pointer in the manager, the spinlock usage was
mostly incorrect as well. Then finally remove all the nonsense checks and
actually add them in the correct locations.

v2: some typo fixes and cleanups suggested by Sunil

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