drm/amd/pp: Delete duplicated interface in hwmgr_func

gfx off support in smu can be via powergate_gfx interface.
so remove the gfx_off_control interface.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
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>