drm/amdkfd: init drm_client with funcs hook

otherwise drm_client_dev_unregister() would try to
kfree(&adev->kfd.client).

Fixes: 18192001 ("drm/amdkfd: Export DMABufs from KFD using GEM handles")
Signed-off-by: Flora Cui <flora.cui@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>