graphics/libdrm: Always enable manpages

No reason to not always include them.
While here change a bit pkg-desc and COMMENT so it better reflects
reality.
Also split amd and radeon, no functional changed intended.

Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 67c3a731efe73e079e402c435c8a93b5921d7971)