drm/amdgpu/powerplay: silence a warning in smu_v11_0_setup_pptable

I think gcc is confused as I don't see how size could be used
uninitialized, but go ahead and silence the warning.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>