drm/udl: optimize udl_compress_hline16 (v2)

The run-length encoding algorithm should compare 16-bit encoded pixel
values instead of comparing raw pixel values. It allows pixels
with similar but different colors to be encoded as repeat pixels, and
thus potentially save USB bandwidth.

Signed-off-by: Haixia Shi <hshi@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Haixia Shi <hshi@chromium.org>
Signed-off-by: Dave Airlie <airlied@gmail.com>