drm/amdgpu: simplify VM update tracking a bit

Store the 64bit sequence directly. Makes it simpler to use and saves a bit
of fence reference counting overhead.

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