drm/amdgpu: Fix two bugs in amdgpu_vm_bo_split_mapping

Off-by-one: last is inclusive, so the maximum is start + max_size - 1
Wrong unit: addr is in bytes, max_size is in pages

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