drm/amdgpu: fix buffer funcs setting order on suspend harder

Part of commit c0358198 ("drm/amdgpu: fix buffer funcs setting order on suspend")
got dropped accidently.  Add it back.

Fixes: c0358198 ("drm/amdgpu: fix buffer funcs setting order on suspend")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>