Revert "drm/amdgpu: use the kernel zone memory size as the max remote memory in amdgpu"

This reverts commit a693e050.

This breaks on systems with huge amounts of system memory as we do not have
enough vram to store the page tables.  Additionally, this is less of an issue
with the recent gtt manager changes.

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