tests/color-icc-output: move gen_ramp_rgb() in the file

Move gen_ramp_rgb() down in the file where the TEST() specific code
begins. This way we first have a big block of fixture setup code which
creates an ICC profile, and the next big block is the actual test client
code. gen_ramp_rgb() belongs with the latter.

This makes the file structure slightly more logical.

This is a pure code move, no changes at all.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>