drm/amdgpu: revert "drm/amdgpu: use AMDGPU_GEM_CREATE_VRAM_CLEARED for VM PD/PTs" v2

Using the standard clear turned out to be to inflexible.

First of all it is executed on the system queue, together with buffer
moves instead on the per VM queue.

And second we need to fill in the page tables with more than just zero.

We keep the new functionality of initializing the PDEs/PTEs with ATC
routing entries intact.

v2: update commit message.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>