drm/amdgpu: check function points valid before use. (v3)

v2: agd: integrate Christian's comments.
v3: print error message if call fails

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>