drm/amd/display: remove unused _calculate_degamma_curve function

We don't use this function anywhere, therefore, remove it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>