[ffmpeg] roll ffmpeg to 8d21d41d8bec5c0b266ee305d1a708dc5c23b594

This CL rolls ffmpeg forward to use a current version of chromium's
third_party/ffmpeg repo. Because fxr/820575 has recently landed,
this CL also introduces the smart_display ffmpeg profile to the
ffmpeg prebuilts.

Test: no behavior change
Change-Id: I2d552e1d950041a583b4b78baaef472ec19bcf92
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/834101
Reviewed-by: Petr Hosek <phosek@google.com>