x11/nvidia-driver, x11/linux-nvidia-libs: Split and distribute distinfo per-slave-ports

Currently, upgrades for legacy versions of x11/nvidia-driver and
x11/linux-nvidia-libs are often delayed, due to the conflicts with
master port and forgotton.

This is usually because master and slave (legacy) ports shares distinfo,
causing races between multiple upgrades, even upgrades for legacy ones
are trivial.

This update splits distinfo into master and each slave (legacy) ports
to avoid races unless master Makefile(s) needs updates for to-be-upgraded
slave ports.

update-distinfo target is removed, you can use the standard
'make makesum' in each port now.

Reviewed by: arrowd
Differential Revision: https://reviews.freebsd.org/D50142