drm/amdgpu/powerplay: add missing breaks in polaris10_smumgr

This was noticed by Gustavo and his -Wimplicit-fallthrough
patches.  However, in this case, I believe we should have breaks
rather than falling though, that said, in practice we should
never fall through in the first place so there should be no
change in behavior.

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