libavdevice/decklink: add support for 10-bit output for Decklink SDI

Can be tested via the following command:

./ffmpeg -i foo.ts -f decklink -vcodec v210 'DeckLink Duo (1)'

Note that the 8-bit support works as it did before, and setting
the pix_fmt isn't required for 10-bit mode.  The code defaults to
operating in 8-bit mode when no vcodec is specified, for backward
compatibility.

Updated to reflect feedback from Marton Balint <cus@passwd.hu>

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>