drm_calloc_large: check right size, check integer overflow, use GFP_ZERO

Previously we would check size instead of size * nmemb, and so would
never hit the vmalloc path.  Also add integer overflow check as in kcalloc,
and allocate GFP_ZERO pages instead of memset()ing them.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>