Resolve compiler warning when highbd is off

The highbd_clip_pixel_add() function is generalized to be used in
the regular 8 bit path. Move its defintions outside the highbd
experimental flag.

This resolves the comiler warning in unit tests when high bit-depth
is turned off.

Change-Id: I90a744adb2381c9bf8476aa2a2bd0c87d9afdf57
2 files changed