drm: Detect no-op drmModeAtomicRequest and return early

If the number of items to process in the request is zero, we can forgo
duplicating, sorting the request and feeding it into the kernel and
instead report success immediately.  This prevents a NULL dereference of
the sorted->items for the no-op request.

Fixes: ed44e0b9
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rob Clark <robclark@freedesktop.org>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>