drm/amdgpu: clean up and unify hw fence handling

drm/amdgpu: clean up and unify hw fence handling

Decouple the amdgpu fence from the amdgpu_job structure.
This lets us clean up the separate fence ops for the embedded
fence and other fences.  This also allows us to allocate the
vm fence up front when we allocate the job.

v2: Additional cleanup suggested by Christian
v3: Additional cleanups suggested by Christian
v4: Additional cleanups suggested by David and
    vm fence fix
v5: cast seqno (David)

Cc: David.Wu3@amd.com
Cc: christian.koenig@amd.com
Tested-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>