amdkfd: use calloc instead of kzalloc to avoid integer overflow

commit 3b0daecf

 upstream.

This uses calloc instead of doing the multiplication which might
overflow.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>