DRM: Replace kmalloc/memset combos with kzalloc

Currently most, if not all, memory allocation in drm_bufs.c is followed by initializing the memory with 0.

Replace the use of kmalloc+memset with kzalloc.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>