drm/amdgpu: add support for new GFX shadow size query

Use the new callback to fetch the data.  Return an error if
not supported.  UMDs should use this query to check whether
shadow buffers are supported and if so what size they
should be.

v2: return an error rather than a zerod structure.
v3: drop GDS, move into dev_info structure.  Data will be
    0 if not supported.
v4: drop local variable r

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>