drm/amdgpu: set ih soft ring enabled flag for vega and navi

software ih ring is enabled in vega10 and navi
ih block by default.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>