[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: NRodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: NStylon Wang <stylon.wang@amd.com>
Signed-off-by: NAlex Hung <alex.hung@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>