drm/amdgpu: detect current GPU memory partition mode

 - Add helpers to detect the current GPU memory partition.
 - Add current memory partition mode sysfs node.

Tested-by: Ori Messinger <Ori.Messinger@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>