libdrm: planetest/atomictest: Fix up build issue missed in rebasing

We dropped commit 5a6afa130317 (BACKPORT: libdrm: atomic mode set)
when moving forward to upstream libdrm, as it should be redundent.

Unfortunately, the API in that commit is slightly different
upstream, and thus the planetest and atomictest tests needed some
updates to use the upstream API

Change-Id: I76cd52f12f6679792a0cedb529973e426932c98c
Signed-off-by: John Stultz <john.stultz@linaro.org>