Add VDENC(lowpower mode) support for QSV h264 and HEVC

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 kerner version >= 4.16)
- HEVC VDENC was supported >= ICE LAKE

use option "-low_power 1" to enable VDENC.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>