drm/amdgpu/VCN: add buffer for indirect SRAM usage

This will be used later for indirect SRAM mode

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>