drm/todo: remove gem_prime_import/export todo

I've done that.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-56-daniel.vetter@ffwll.ch