low_power mode will use a fixed HW engine (SFC), thus can offload EU usage.
high quality mode will take EU usage (AVS sampler).

Performance and EU usage (Render usage) comparsion on Intel(R) Xeon(R) CPU E3-1225 v5 @ 3.30GHz:

High quality mode : ffmpeg -hwaccel qsv -c:v h264_qsv -i bbb_sunflower_1080p_30fps_normal_2000frames.h264 \
-vf scale_qsv=w=1280:h=736:mode=hq -f null -
fps=389
RENDER usage: 28.10 (provided by MSDK metrics_monitor)

Low Power mode: ffmpeg -hwaccel qsv -c:v h264_qsv -i ~/bbb_sunflower_1080p_30fps_normal_2000frames.h264 \
-vf scale_qsv=w=1280:h=736:mode=low_power -f null -
fps=343
RENDER usage: 0.00

Low power mode (SFC) may be disabled if not supported by
MSDK/Driver/HW, and replaced by AVS mode interanlly.
Signed-off-by: Zhong Li <zhong.li@intel.com>