Add estimate of how much vram we need to reserve for RAS
when caculating the total available vram.

v2: apply the change to MP0 v13_0_2 and v13_0_14
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>