drm/amdgpu: remove RAS GFX injection for gfx_v9_4/gfx_v9_4_2

No special requirement in RAS injection for the two versions, switch to
use default injection interface.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Stanley.Yang <Stanley.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>