drm/vgem: Ditch attach trickery in the fence ioctl

It looks like this was done purely to get a consistent place to look
up the reservation object pointer. With the drm_prime.c helper code
now also setting gem_object->resv for imported objects we can just use
that pointer directly, instead of first ensuring a dma-buf exists.

v2: Note that I screwed up the patch ordering, hence why this needed
a Fixes: tag - CI spotted the broken intermediate state.

Fixes: 5eab9987 ("drm/vgem: Drop drm_gem_prime_export")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-49-daniel.vetter@ffwll.ch