GCC requires the argument to vec_splat_u32 to be a literal.  The easiest
way to accomplish this is to change 'shift' to be const in scale (as it
is in the transform routine above), and convert both routines to be
inline.  This way, GCC can coerce the values to literals.

Tested on a 970 (Apple G5) and POWER9 (Talos II); passed fate and played
a clip of Big Buck Bunny correctly.

Fixes ticket #7048
Signed-off-by: A. Wilcox <AWilcox@Wilcox-Tech.com>