drm/amdgpu: No need to check gfxoff status after enable gfxoff feature

smu_send_smc_msg(smu, SMU_MSG_AllowGfxOff) Just turn on a switch.

As to when GPU get into "GFXoff" will be up to drawing load.

So we can not sure which state GPU should be in after enable gfxoff
feature.

Signed-off-by: chen gong <curry.gong@amd.com>
Acked-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>