drm/amd/pp: Rename enable_per_cu_power_gating to powergate_gfx

keep consistent with powergate_uvd/vce/mmhub

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