console: untangle gfx & txt updates

Stop abusing displaysurface fields for text mode displays.
(bpp = 0, width = cols, height = lines).

Add flags to displaystate indicating whenever text mode display
(curses) or gfx mode displays (sdl, vnc, ...) are present.

Add separate displaychangelistener callbacks for text / gfx mode
resize & updates.

This allows to enable gfx and txt diplays at the same time and also
paves the way for more cleanups in the future.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/omap_lcdc.c

@@ -219,7 +219,7 @@ static void omap_update_display(void *opaque)
                                draw_line, omap_lcd->palette,
                                &first, &last);
     if (first >= 0) {
-        dpy_update(omap_lcd->state, 0, first, width, last - first + 1);
+        dpy_gfx_update(omap_lcd->state, 0, first, width, last - first + 1);
     }
     omap_lcd->invalidate = 0;
 }