Fix kernel memory disclosure in drm_infobufs

drm_infobufs() has a structure on the stack, fills it out and copies it
to userland. There are 2 elements in the struct that are not filled out
and left uninitialized. This will leak uninitialized kernel stack data
to userland.

Submitted by: Domagoj Stolfa <ds815@cam.ac.uk>
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 1 day
Security: Kernel memory disclosure (798)