multimedia/libva-intel-driver: deprecate due to dead upstream

https://github.com/intel/intel-vaapi-driver/commit/fd727a4e9cb8

Not blindly recommending multimedia/libva-intel-media-driver:
- lacks support for pre-Broadwell (Gen < 8) GPUs
- lacks VP9 decode on Skylake (Gen9)
- lacks VP9 encode on Kabylake (Gen9.5)

Deprecation is for existing users to exercise caution decoding videos
from untrusted sources since the vendor no longer promises security fixes.