The argument to vec_splat_u16 must be a literal. By making the
function always inline and marking the arguments const, gcc can
turn those into literals, and avoid build errors like:

swscale_vsx.c:165:53: error: argument 1 must be a 5-bit signed literal

Fixes #7861.
Signed-off-by: Daniel Kolesa <daniel@octaforge.org>
Signed-off-by: Lauri Kasanen <cand@gmx.com>