agp: Add bridge parameter documentation

This patch add documentation about the bridge parameter in several
function.

This will fix the following build warning:
drivers/char/agp/generic.c:220: warning: No description found for parameter 'bridge'
drivers/char/agp/generic.c:364: warning: No description found for parameter 'bridge'
drivers/char/agp/generic.c:1283: warning: No description found for parameter 'bridge'

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/1574324085-4338-5-git-send-email-clabbe@baylibre.com