drm/amdgpu: all vram is visible for APU (v2)

missed in gmc9.

v2: squash in build fix (Rex)

Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>