amd/common: unify cube map coordinate handling between radeonsi and radv

amd/common: unify cube map coordinate handling between radeonsi and radv

Code is taken from a combination of radv (for the more basic functions,
to avoid gallivm dependencies) and radeonsi (for the new and improved
derivative calculations).

v2: add 0.5 offset to tex coords only after derivative calculation

v3:
- really only touch the first three coordinates
- rebase on the removal of the 1.5 --> 0.5 offset change

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> (v2)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>