drm/amd/amdgpu: Cleanup gmc_v9_0_suspend()

Even though fini returns 0 always it could theoretically
fail in the future.  Might as well return it instead of 0.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>