drm/amdgpu: add Vega20 PSP ASD firmware loading

Add PSP ASD firmware loading on Vega20. Not sure why
this was missing before.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org