drm/amdgpu: drop AMDGPU_FENCE_SIGNALED_SEQ

It's causing issues with VMID handling and comparing the
fence value two times actually doesn't make handling faster.

Port of radeon commit "d6d5c5b8".

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