xen/gntdev: Prepare to dynamic dma-buf locking specification

Prepare gntdev driver to the common dynamic dma-buf locking convention
by starting to use the unlocked versions of dma-buf API functions.

Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221017172229.42269-13-dmitry.osipenko@collabora.com