drm/amd/display: Drop DCN for DP2.x logic

[Why & How]
DCN guard is not necessary for DP2.x relevant logic.
Drop them.

v2: squash in fix for misplaced #endif (Alex)

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Jerry Zuo <Jerry.Zuo@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>