GPU-DRM: Replace a kzalloc() call by kcalloc() in drm_legacy_addbufs_sg()

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

WARNING: Prefer kcalloc over kzalloc with multiply

Thus fix the affected source code place.

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/b340d1ca-f114-a523-e6d1-afe7fd6c3826@users.sourceforge.net