add amd vaapi ffmpeg docs

arch/gaming.md

@@ -209,8 +209,8 @@ Then add `mangohud env MANGOHUD_CONFIGFILE=/home/ducoterra/.config/MangoHud/Mang
 pacman -S ffmpeg libmfx intel-media-sdk
 
 ffmpeg \
--hwaccel qsv \              
+-hwaccel qsv \
--c:v hevc_qsv \                    
+-c:v hevc_qsv \
 -hwaccel_output_format qsv \
 -i input.mkv \
 -c:v hevc_qsv \
@@ -218,4 +218,22 @@ ffmpeg \
 output.mp4
 ```
 
-### AMD 7900xtx
+### AMD 7900xtx
+
+<https://wiki.archlinux.org/title/FFmpeg#VA-API>
+
+```bash
+sudo pacman -S ffpmeg mesa libva-mesa-driver
+
+reboot
+
+ffmpeg \
+-vaapi_device /dev/dri/renderD128 \
+-i input.mp4 \
+-vf 'format=nv12,hwupload' \
+-c:v hevc_vaapi \
+-f mp4 \
+-rc_mode 1 \
+-qp 25 \
+output.mp4
+```