drm/amdgpu/display: Protect some functions with CONFIG_DRM_AMD_DC_DCN

Protect remove_hpo_dp_link_enc_from_ctx() and release_hpo_dp_link_enc()
with CONFIG_DRM_AMD_DC_DCN as the functions are only called from code
that is protected by CONFIG_DRM_AMD_DC_DCN. Fixes build fail with
-Werror=unused-function.

Fixes: 9b0e0d433f74 ("drm/amd/display: Add dependant changes for DCN32/321")
Reported-by: Stephen Rothwell <[e-mail]>
Reviewed-by: Harry Wentland <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: Aurabindo Pillai <[e-mail]>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

···
 	res_ctx->hpo_dp_link_enc_ref_cnts[enc_index]++;
 }
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 static inline void release_hpo_dp_link_enc(
 		struct resource_context *res_ctx,
 		int enc_index)
···
 	ASSERT(res_ctx->hpo_dp_link_enc_ref_cnts[enc_index] > 0);
 	res_ctx->hpo_dp_link_enc_ref_cnts[enc_index]--;
 }
+#endif
 
 static bool add_hpo_dp_link_enc_to_ctx(struct resource_context *res_ctx,
 		const struct resource_pool *pool,
···
 	return pipe_ctx->link_res.hpo_dp_link_enc != NULL;
 }
 
+#if defined(CONFIG_DRM_AMD_DC_DCN)
 static void remove_hpo_dp_link_enc_from_ctx(struct resource_context *res_ctx,
 		struct pipe_ctx *pipe_ctx,
 		struct dc_stream_state *stream)
···
 		pipe_ctx->link_res.hpo_dp_link_enc = NULL;
 	}
 }
+#endif
 
 /* TODO: release audio object */
 void update_audio_usage(