drm/amdgpu: fix userptr put_page handling

Move calling put_page into the unpopulate callback. Otherwise we mess up the pages
reference count when it is unbound multiple times.

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>