drm/amdgpu: force exit gfxoff on sdma resume for rmb s0ix

In the s2idle stress test sdma resume fail occasionally,in the
failed case GPU is in the gfxoff state.This issue may introduce
by firmware miss handle doorbell S/R and now temporary fix the issue
by forcing exit gfxoff for sdma resume.

Signed-off-by: Prike Liang <Prike.Liang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org