graphics/nvidia-drm-kmod[-devel]: Fix dependency for 15.0 and later

At commit ports 324e88dfe426, graphics/drm-kmod switches from
graphics/drm-61-kmod to graphics/drm-66-kmod for OSVERSION 1500031 and
later.

Chase the change not to mixup LinuxKPI 6.1 and 6.6 within
graphics/drm-kmod and graphics/nvidia-drm-kmod[-devel] on 15.0 and
later.

MFH: 2025Q4
Differential Revision: https://reviews.freebsd.org/D53196