drm/amd/powerplay: Tidy up cz_dpm_powerdown_vce()

Use PP_CAP macro.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>