drm/amdgpu: handle CPU access for split VRAM buffers (v2)

This avoids merging them together on page fault.

v2: squash in 64-bit division fix

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>