graphics/drm-fbsd12.0-kmod: Remove firmware dep

drm-*-kmod ports should not depend on firmware, so users are free to
only install the firmware packages they actually need. Pulling in all
firmwares should only happen in graphics/drm-kmod metaport.

PR: 266301
Approved by: x11 (manu), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36526