x11/nvidia-driver-[204,340]: Fix build on CURRENT

Commit 9302fb05a0c6 "x11/nvidia-driver: Fix build after kva_layout"
accidentally broke builds for x11/nvidia-driver-304 and
x11/nvidia-driver-340.

Fix builds for these 2 slave (child) ports.
Other slave ports are not affected with this breakage.

Reported by: pkg-fallout
Differential Revision: https://reviews.freebsd.org/D51888