drm/amdgpu: Remove unecessary dma_fence_ops

dma_fence_default_wait is the default now.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Monk Liu <Monk.Liu@amd.com>
Cc: pding <Pixel.Ding@amd.com>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Evan Quan <evan.quan@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kees Cook <keescook@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503142603.28513-6-daniel.vetter@ffwll.ch