GPU-DRM: Replace two kzalloc() calls by kcalloc() in drm_legacy_addbufs_pci()

The script "checkpatch.pl" can point information out like the following.

WARNING: Prefer kcalloc over kzalloc with multiply

Thus fix the affected source code places.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/51f88f44-130d-48ac-f531-32ca9ae3919f@users.sourceforge.net