drm/amd/display: Refactor to remove diags specific rgam func

drm/amd/display: Refactor to remove diags specific rgam func

[Why]
It has duplicate code for building regamma curve

[How]
Remove the duplicate code and use the same function for building regamma

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Reviewed-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>