drm/amd/powerplay: allow slow switch only if NBPState enabled v2

Otherwise there may be potential SMU performance issues.

v2: fix commit description and coding style

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