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.

Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alexey Skidanov <Alexey.skidanov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>