drm/vram-helper: turn on PRIME import/export

We don't populate drm_driver.gem_prime_import_sg_table so only
DMA-BUFs exported from our own device can be imported. We don't
populate drm_gem_object_funcs.get_sg_table so DMA-BUFs cannot be
imported into another device. Still, this is useful to user-space
to share buffers between processes and between API boundaries
(e.g. wlroots hard-requires PRIME import/export support).

v2: expand commit message

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tian Tao <tiantao6@hisilicon.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Hans de Goede <hdegoede@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230302143502.500661-2-contact@emersion.fr