drm/amdgpu/powerplay: fix missing break in switch statements

There are several switch statements that are missing break statements.
Add missing breaks to handle any fall-throughs corner cases.

Detected by CoverityScan, CID#1457175 ("Missing break in switch")

Fixes: 18aafc59 ("drm/amd/powerplay: implement fw related smu interface for iceland.")
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>