drm/prime: Unconditionally set up the prime file private

It's tiny, already embedded, and setup/teardown cost is trivial.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-5-daniel.vetter@ffwll.ch