cgem: rework hardware quirk detection

Rather than doing these checks based on the detected hardware variant, allow
quirks to be specified as a set of flags for each compatible string.
This simplifies adding support for new compatible hardware.

Reviewed by: mhorne
MFC after: 1 week
Sponsored by: Conclusive Engineering
Differential Revision: https://reviews.freebsd.org/D34764

(cherry picked from commit 66cc0c61b023a5d8ae1c05897bfaf7726be40934)