drm/amd/display: Proper de-allocation of OPP

- refactor opp_destroy functions to dce common file
  - fixes memory leak, dce specific variations didn't free regamma_params
- remove unused dce110_regamma structure

Signed-off-by: Joshua Aberback <Joshua.Aberback@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>