src/meta-raspberrypi:rpidistro-ffmpeg: remove --enable-v4l2-request flag

When building with latest OE-Core master. Running into
bellow issue. After further research issue appears to
be similar to

bootlin/libva-v4l2-request#35

| src/libavcodec/hevc-ctrls-v2.h:59:8: error: redefinition of 'struct
v4l2_ctrl_hevc_sps'
|    59 | struct v4l2_ctrl_hevc_sps {
|       |        ^~~~~~~~~~~~~~~~~~
| In file included from
/../../../recipe-sysroot/usr/include/linux/videodev2.h:65,
|                  from src/libavcodec/hevc-ctrls-v2.h:14:

Disable ffmpeg libva-v4l2-request so that master builds
and until better investigation can be done to fix
issue.

Signed-off-by: Vincent Davis Jr <[email protected]>