dxva2: fix incorrect return value

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
dxva2: remove useless variable

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
dxva2: check number of surfaces to allocate is not larger than the upper bound

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
va: pass AVCodecContext rather than only the codec ID (outer part)

This should perhaps be const... but libavcodec seems to require
non-const even when const ought to be enough :-(
va: pass AVCodecContext pointer (inner part)
vdpau_avcodec: simplify with av_vdpau_get_profile()
vdpau_avcodec: fix mismatched alloc/free
hwdummy: update to libavcodec 55.26.0

(see also fccfbd2fd1f6e91ecc3888c361348d35893140fb)