drm/amdgpu: make non-NULL out fence mandatory

drm/amdgpu: make non-NULL out fence mandatory

amdgpu_ttm_copy_mem_to_mem has a single caller, make sure the out
fence is non-NULL to simplify the code.
Since none of the pointers should be NULL, we can enable
__attribute__((nonnull))__.

While at it make the function static since it's only used from
amdgpuu_ttm.c.

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>