Revert "drm/prime: Unexport helpers for fd/handle conversion"

This reverts commit 71a7974a

.

These helper functions are needed for KFD to export and import DMABufs
the right way without duplicating the tracking of DMABufs associated with
GEM objects while ensuring that move notifier callbacks are working as
intended.

CC: Christian König <christian.koenig@amd.com>
CC: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>