vdpau: Fix VC-1 interlaced mode

VDPAU expects the bitstream value (0, 2 or 3).
libavcodec uses an enum (0, 1 or 2).

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: Diego Biurrun <diego@biurrun.de>