drm/amdgpu: enable Vangogh VCN indirect sram mode

So that uses PSP to initialize HW.

Fixes: 0c2c02b6 ("drm/amdgpu/vcn: add firmware support for dimgrey_cavefish")
Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>