drm/amdgpu: adjust fence driver enable sequence

Fence driver was enabled per ring when sw init on per IP block before.
Change to enable all the fence driver at the same time after
amdgpu_device_ip_init finished.
Rename some function related to fence to make it reasonable for read.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>