drm/hisilicon: Drop drm_gem_prime_export/import

They're the default.

Aside: Would be really nice to switch the others over to
drm_gem_object_funcs.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Xinliang Liu <z.liuxinliang@hisilicon.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-17-daniel.vetter@ffwll.ch