drm/amdgpu: use a 64bit interval tree for VM management v2

This only makes a difference for 32-bit systems. The idea is to have a
fixed virtual address space size with 4-level page tables and to
minimize differences between 32 and 64-bit systems.

v2: Update commit message.

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