Fix a common vp8 decoding failure.

Many vp8 clips cannot decode if hw_frames_ctx is enabled, reporting
"Error during QSV decoding.: incompatible video parameters (-14)".

It is due to mfx.FrameInfo.Width/Height not matching coded_w/coded_h.

See: avconv -hwaccel qsv -init_hw_device qsv -c:v vp8_qsv -i vp8-test-vectors-r1/vp80-00-comprehensive-001.ivf
-vf "hwdownload,format=nv12" -pix_fmt yuv420p -f md5 -
Signed-off-by: Zhong Li <zhong.li@intel.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>