drm/amdgpu: revert "reserve backup pages for bad page retirment"

As noted during the review this approach doesn't make sense at all.

We should not apply any limitation on the VRAM applications can use inside the kernel.

If an application or end user wants to reserve a certain amount of VRAM for bad pages handling we should do this in the upper layer.

This reverts commit f89b881c.

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