drm/amdgpu: Update BO memory accounting to rely on allocation flag

Accounting system to track amount of available memory (system, TTM
and VRAM of a device) relies on BO's domain. The change is to rely
instead on allocation flag indicating BO type - VRAM, GTT, USERPTR,
MMIO or DOORBELL

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>