agp: Documentation fixes

Fix compiler warnings

  drivers/char/agp/backend.c:68: warning: Function parameter or member 'pdev' not described in 'agp_backend_acquire'
  drivers/char/agp/backend.c:93: warning: Function parameter or member 'bridge' not described in 'agp_backend_release'

by adding the necessary documentation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201114645.15384-4-tzimmermann@suse.de