drm/amdkfd: fix some range checks in address watch ioctl

buf_size_in_bytes must be large enough to hold ->num_watch_points and
watch_mode so I have added a sizeof(int) * 2 to the minimum size.

Also we have to subtract sizeof(*args) from the max args_idx limit so
that it matches the allocation.  Also I changed a > to >= for the last
compare.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>