drm/amdgpu: Create generic DF struct in adev

The only data fabric information the adev struct currently
contains is a function pointer table. In the near future,
we will be adding some cached DF information into adev. As
such, this patch creates a new amdgpu_df struct for adev.
Right now, it only containst the old function pointer table,
but new stuff will be added soon.

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