drm/amdgpu: Implement new dummy vram manager

This adds dummy vram manager to support ASICs that do not have a
dedicated or carvedout vram domain.

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>