drm/amdgpu: stop joining VM PTE updates

This isn't beneficial any more since VRAM allocations are now split
so that they fits into a single page table.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>