drm/amdgpu: use GMC v9 KIQ workaround only for the GFXHUB

The MMHUB is not affected by this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emily Deng <Emily.Deng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>