drm/amd/display: Cleanup pwl structs and names
- Rename "plane gamma/degamma non-linear" to just "non-linear" since gamma/plane are self explanatory
- Remove degamma lut definition, and reuse gamma lut definition since gamma/degamma pwl is the same
Signed-off-by: Bhawanpreet Lakha Bhawanpreet.Lakha@amd.com