drm/amdgpu: revert "Don't change preferred domian when fallback GTT v6"

This reverts commit 7d1ca1325260a9e9329b10a21e3692e6f188936f.

Makes fallback handling to complicated. This is just a feature for the
GEM interface and shouldn't leak into the core BO create function.

The intended change to preserve the preferred domains is implemented in
a follow up patch.

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