multimedia/libva: fix default LIBVA_DRIVER_NAME with nvidia-drm.ko

In order to support both libva-vdpau-driver and libva-nvidia-driver
on NVIDIA GPUs let libva-drm get driver name from kernel DRM driver.
libva-nvidia-driver uses DRI2 instead of DRM, so not affected.

libva info: Trying to open /usr/local/lib/dri/nvidia_drv_video.so
libva info: va_openDriver() returns -1
vaInitialize failed with error code -1 (unknown libva error),exit

Reported by: rhurlin
Regressed by: https://github.com/intel/libva/commit/5dae76daad3f