drm/amd/display: Drop unnecessary DCN guards

[Why]
DM is littered with DCN guards leading to frequent
breakages on non-DCN builds when new code is added.

[How]
Remove all guards that are not needed.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>