GPU-DRM: Use kmalloc_array() in drm_legacy_addbufs_pci()

A multiplication for the size determination of a memory allocation
indicated that an array data structure should be processed.
Thus use the corresponding function "kmalloc_array".

This issue was detected by using the Coccinelle software.

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/b71c8fee-8e84-9f1b-8569-f1ae8b879cc5@users.sourceforge.net