GMC in the GPU memory controller.

v1: add place holder and initial basic implementation (Ray)
v2: retire unused amdgpu_gart_set_defaults (Hawking)
v3: re-work get_vm_pde function (Hawking)
v4: replace legacy amdgpu_vram/gtt_location with
    amdgpu_gmc_vram/gtt_location (Hawking)
v5: squash in updates (Alex)
v6: use get_vbios_fb_size (Alex)
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>