lavc/qsvenc: add VDENC support for H264

lavc/qsvenc: add VDENC support for H264

Add VDENC(lowpower mode) support for QSV H264

It's an experimental function(like lowpower in vaapi) with
some limitations:
- CBR/VBR require HuC which should be explicitly loaded via i915
module parameter(i915.enable_guc=2 for linux kernel version >= 4.16)

Use option "-low_power 1" to enable VDENC.
Add in dump_video_param() to show the status of VDENC in runtime log.

Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Zhong Li <zhong.li@intel.com>