drm/amd/display: add more link_hwss types and method to decide which one

[why]
as we add more link_hwss we are making a generic way to determine which type
of link_hwss we should use.
Later on we may think of introduce a link policy layer. it could be a thin layer
that decide the type of link_hwss we use. So instead of passing in link and link_res
we can just pass in link_policy and swtich based on link_policy->get_link_hwss_type.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@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>

drivers/gpu/drm/amd/display/dc/inc/link_hwss.h

@@ -78,6 +78,9 @@ struct fixed31_32;
 struct pipe_ctx;
 
 struct dc_link_hwss {
+	/* you must define a dummy implementation and assign the function to
+	 * dummy_link_hwss if you don't want to check for NULL pointer
+	 */
 	void (*set_throttled_vcp_size)(struct pipe_ctx *pipe_ctx,
 			struct fixed31_32 throttled_vcp_size);