drm/amd/amdgpu: Fix uninitialized variable warnings

Return 0 to avoid returning an uninitialized variable r.

Cc: stable@vger.kernel.org
Fixes: 230dd6bb ("drm/amd/amdgpu: implement mode2 reset on smu_v13_0_10")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>