drm/amd/display: remove redundant CONFIG_DRM_AMD_DC_DCN in amdgpu_dm

[Why & How]
CONFIG_DRM_AMD_DC_DCN is used by pass the compilation failures, but DC
code should be OS-agnostic.

This patch fixes it by removing unnecessasry CONFIG_DRM_AMD_DC_DCN
in amdgpu_dm directory.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>