graphics/intel-backlight: change to FreeBSDDesktop github to support newer Intel GPUs. Take maintainership over on behalf of FreeBSDDesktop team.

Reviewed by: swills (mentor), grembo (previous maintainer)
Approved by: swills (mentor), grembo (previous maintainer)
Obtained from: FreeBSDDesktop
Differential Revision: https://reviews.freebsd.org/D12196