agp: Handle multiple devices more gracefully

Currently agp(4) effectively assumes that only one driver instance
exists, as the generic attach routine attempts to create /dev/agpgart
and triggers a panic if it already exists. Instead, handle this
situation by creating /dev/agpgart<unit> and making /dev/agpgart an
alias of /dev/agpgart0 for compatibility.

PR: 187015
Reviewed by: imp, kib
Tested by: Yoshihiro Ota <ota@j.email.ne.jp> (earlier version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33068