drm/amdkfd: Allocate gart memory using new interface
This patch changes the calls to allocate the gart memory for amdkfd from the
old interface (radeon_sa) to the new one (kfd_gtt_sa)
The new gart sub-allocator is initialized with chunk size equal to 512 bytes.
This is because the KV MQD is 512 Bytes and most of the sub-allocations are
MQDs.