drm/amd/display: Clean up code warnings

- Fix constness of string parameters
- Fix zero-initialization of structs which cannot take a 0 literal as the
  first field, for example enums
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/modules/inc/mod_stats.h

@@ -53,7 +53,7 @@ void mod_stats_dump(struct mod_stats *mod_stats);
 void mod_stats_reset_data(struct mod_stats *mod_stats);
 
 void mod_stats_update_event(struct mod_stats *mod_stats,
-		char *event_string,
+		const char *event_string,
 		unsigned int length);
 
 void mod_stats_update_flip(struct mod_stats *mod_stats,