drm/amdgpu: Deactivate SW interrupt fallback in amdgpu_fence_process v2

Deactivate SW interrupt fallback when all emited fences are completed.
Also switch interrupt SW fallback message from INFO to WARN.

v2: shorten the warning message a bit and only re-activate the timer during
processing if it was already activated before. (Christian)

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Suggested-by: Christian Konig <Christian.Koenig@amd.com>
Reviewed-and-Tested-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>