An alternative to MOZ_DRM_DEVICE for libva "LIBVA_DRM_DEVICE"

We have `LIBVA_DRIVER_NAME`, and `LIBVA_DRIVERS_PATH` to find libva driver libraries. But there is no replacement for `MOZ_DRM_DEVICE` https://bugzilla.mozilla.org/show_bug.cgi?id=1751710 (`LIBVA_DRM_DEVICE=/dev/dri/renderD129` or `LIBVA_DRM_DEVICE=/dev/dri/card1`) so that we can specify the right default path to drm and not use the automatic search which may fail https://github.com/intel/libva-utils/blob/2.19.0/common/va_display_drm.c#L49-L51 with an undefined error #750.

> /dev/dri/renderD128 - NVIDIA
> /dev/dri/renderD129 - Intel HD Graphics (KBL)

> $ env | grep LIBVA_
> LIBVA_DRIVER_NAME=iHD
> $ vainfo
> DRM_IOCTL_VERSION, unsupported drm device by media driver: nvid
> libva error: /usr/lib/dri/iHD_drv_video.so init failed

> $ env | grep LIBVA_
> LIBVA_DRIVER_NAME=iHD
> $ env LIBVA_DRIVER_NAME=iHD vainfo --display drm --device /dev/dri/renderD128
> Trying display: drm
> DRM_IOCTL_VERSION, unsupported drm device by media driver: nvid
> DRM_IOCTL_VERSION, unsupported drm device by media driver: nvid
> libva error: /usr/lib/dri/iHD_drv_video.so init failed
> __vaInitialize failed with error code 18 (invalid parameter),exit__

> $ env | grep LIBVA_
> LIBVA_DRIVER_NAME=iHD
> $ env LIBVA_DRIVER_NAME=nvidia vainfo --display drm --device /dev/dri/renderD129
> Trying display: drm
> libva error: /usr/lib/dri/nvidia_drv_video.so init failed
> __vaInitialize failed with error code 1 (operation failed),exit__

> $ env | grep LIBVA_
> LIBVA_DRIVER_NAME=iHD
> $ google-chrome-stable
> __libva error: vaGetDriverNames() failed with unknown libva error__
> DRM_IOCTL_VERSION, unsupported drm device by media driver: nvid
> DRM_IOCTL_VERSION, unsupported drm device by media driver: nvid
> libva error: /usr/lib/dri/iHD_drv_video.so init failed

https://github.com/intel/libva/issues/677