drm/qxl: use kmalloc_array to alloc reloc_info in qxl_process_single_command

This avoids integer overflows on 32bit machines when calculating
reloc_info size, as reported by Alan Cox.

Cc: stable@vger.kernel.org
Cc: gnomes@lxorguk.ukuu.org.uk
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>