agp: remove unused variable size in agp_generic_create_gatt_table

This patch fix the following warning:
drivers/char/agp/generic.c:853:6: attention : variable ‘size’ set but not used [-Wunused-but-set-variable]
by removing the unused variable size in agp_generic_create_gatt_table

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/1574324085-4338-2-git-send-email-clabbe@baylibre.com