drm/i915: Fix up broken merge

Maxime didn't really compile-test this :-/

We need to re-apply

commit e4fa8457
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jun 14 22:35:25 2019 +0200

    drm/prime: Align gem_prime_export with obj_funcs.export

plus make sure i915_gem_dma_buf.c doesn't get zombie-resurrect. It
moved in

commit 10be98a7
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue May 28 10:29:49 2019 +0100

    drm/i915: Move more GEM objects under gem/

v2: Remember the selftests (Chris).

Fixes: 03b0f2ce

 ("Merge v5.3-rc1 into drm-misc-next")
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722213759.26612-1-daniel.vetter@ffwll.ch