drm/amd/powerplay: Move fallthrough; into containing #ifdef/#endif

The automated conversion of /* fallthrough */ comments converted
a comment outside of an #ifdef/#endif case block that should be
inside the block.

Move the fallthrough inside the block to silence the warning.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>