agp: remove unused variable num_segments

This patch fix the following build warning:
warning: variable 'num_segments' set but not used [-Wunused-but-set-variable]

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1574324085-4338-4-git-send-email-clabbe@baylibre.com