debug: dont update asm view in the start_hook()

The curpc can be in the middle of an instruction being decoded, so
it is not pertinent to recompute the disassembly starting from
this address.

src/emu/debug/debugcpu.c

@@ -1731,7 +1731,7 @@ void device_debug::start_hook(const attotime &endtime)
 		// check for periodic updates
 		if (&m_device == global->visiblecpu && osd_ticks() > global->last_periodic_update_time + osd_ticks_per_second()/4)
 		{
-			m_device.machine().debug_view().update_all();
+			m_device.machine().debug_view().update_all_except(DVT_DISASSEMBLY);
 			m_device.machine().debug_view().flush_osd_updates();
 			global->last_periodic_update_time = osd_ticks();
 		}

src/emu/debug/debugvw.c

@@ -397,6 +397,20 @@ void debug_view_manager::free_view(debug_view &view)
 }
 
 
+//-------------------------------------------------
+//  update_all_except - force all views to refresh
+//  except one
+//-------------------------------------------------
+
+void debug_view_manager::update_all_except(debug_view_type type)
+{
+	// loop over each view and force an update
+	for (debug_view *view = m_viewlist; view != NULL; view = view->next())
+		if (type == DVT_NONE || type != view->type())
+			view->force_update();
+}
+
+
 //-------------------------------------------------
 //  update_all - force all views to refresh
 //-------------------------------------------------

src/emu/debug/debugvw.h

@@ -240,6 +240,7 @@ public:
 
 	// update helpers
 	void update_all(debug_view_type type = DVT_NONE);
+	void update_all_except(debug_view_type type = DVT_NONE);
 	void flush_osd_updates();
 
 private: