vl: reintroduce PIPE_VIDEO_CAP_PREFERED_FORMAT

Create the video buffers in the format the driver preffers.
This temporary creates problems with decoder less VDPAU video playback.

Signed-off-by: Christian König <deathsimple@vodafone.de>

src/gallium/drivers/softpipe/sp_screen.c

@@ -201,6 +201,8 @@ softpipe_get_video_param(struct pipe_screen *screen,
    case PIPE_VIDEO_CAP_MAX_WIDTH:
    case PIPE_VIDEO_CAP_MAX_HEIGHT:
       return vl_video_buffer_max_size(screen);
+   case PIPE_VIDEO_CAP_PREFERED_FORMAT:
+      return PIPE_FORMAT_NV12;
    default:
       return 0;
    }