disable vdpau in ffmpeg

Fixes an issue where Ring would sometimes fail to build because
of an undefined reference to "vdp_device_create_x11".

VDPAU should be re-enabled once it is implemented.

Change-Id: Idc420931674f7455a6ae561afa9da6e5808d78fb
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

contrib/src/ffmpeg/rules.mak

@@ -94,6 +94,7 @@ endif
 
 ifdef HAVE_LINUX
 FFMPEGCONF += \
+	--disable-vdpau \
 	--enable-vaapi \
 	--enable-hwaccel=h264_vaapi \
 	--enable-hwaccel=mpeg4_vaapi \