agp: move AGPGART_MINOR to include/linux/miscdevice.h

This patch move the define for AGPGART_MINOR to include/linux/miscdevice.h.
It is better that all minor number definitions are in the same place.

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/1574324085-4338-3-git-send-email-clabbe@baylibre.com