drm/amdgpu: when resume failed, return error to avoid system hang.

Continuing if the GPU fails to resume will end in pain.

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