xf86drmMode: smoke-test the atomic API

As going through the modetest patches for atomic support I've noticed
that if we pass NULL for the drmModeAtomicReqPtr argument we'll crash.

So let's handle things appropriately if the user forgot to check the
return value of drmModeAtomicAlloc and drmModeAtomicDuplicate or made a
typo somewhere along the way.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rob Clark <robclark@freedesktop.org>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>