drm/amd/powerplay: Enable "disable dpm" feature to support swSMU debug (v2)

add pm_enabled to control the dpm off/on.
v2: Directly return 0 to replace return ret and merge some check code.

Signed-off-by: Chengming Gui <Jack.Gui@amd.com>
Reviewed-by: Hawking Zhang <hawking.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>