drm/amd/display: Add comment where CONFIG_DRM_AMD_DC_DCN macro ends

Trivial patch which adds a comment for macro
endif's in amdgpu_dm.c

Reviewed-by: Ariel Bernstein <Eric.Bernstein@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Anson Jacob <Anson.Jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>