Graphics: Switch to 800x600x32 as default mode

We have stayed at 800x600x15 as default graphics mode for the last 9 years.
If there ever was a reason to be there, surely nobody remembers it.

However, recently non-Linux PPC guests started to show bad effects on 15 bit
color mode. They do work just fine with 32 bits however.

So let's switch to 32 bit color as the default graphic mode.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Alexander Graf <agraf@suse.de>

arch_init.c

@@ -65,7 +65,7 @@ int graphic_depth = 8;
 #else
 int graphic_width = 800;
 int graphic_height = 600;
-int graphic_depth = 15;
+int graphic_depth = 32;
 #endif