drm/amdgpu: Make sure to reserve BOs before adding or removing

BOs need to be reserved before they are added or removed, so ensure that
they are reserved during kfd_mem_attach and kfd_mem_detach

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>