gl-renderer: consolidate linpow sampling GLSL

This is pure refactoring.

Ease readability by reducing code duplication between pre and post curve
linpow handling.

While at it, define symbols for the counts. This patch converts only
linpow. Powlin are converted in follow-up.

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