monitor-manager: Add logging to enabling and disabling HDR mode

It's hard to tell why turning on HDR mode failed without these log
messages. It could be missing support in the sink (EDID/DisplayID) or
missing support in the driver/display hardware (connector properties) or
just a failure turning it on.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3251>

src/backends/meta-output.h

@@ -162,6 +162,8 @@ gboolean
 meta_tile_info_equal (MetaTileInfo *a,
                       MetaTileInfo *b);
 
+const char * meta_output_colorspace_get_name (MetaOutputColorspace color_space);
+
 #define META_TYPE_OUTPUT_INFO (meta_output_info_get_type ())
 META_EXPORT_TEST
 GType meta_output_info_get_type (void);