drm/amd/display: Rework dcn10_stream_encoder header

This commit remove some unused code and also rename one of the define.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>