drm/amd/powerplay: correct Navi10 VCN powergate control (v2)

No VCN DPM bit check as that's different from VCN PG. Also
no extra check for possible double enablement/disablement
as that's already done by VCN.

v2: check return value of smu_feature_set_enabled

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>