ubb-vga: gently prepare the code for sharing mode information

This is tricky: if we just make "mode" global, the whole timing may
fall apart, with the DMA locking up. So let's take small bisectable
steps to get there ...

- ubb-vga.c (struct mode), ubb-vga.h: moved mode entry declaration to
  ubb-vga.h
- ubb-vga.c (mode_db): moved up

ubb-vga/ubb-vga.h

@@ -23,6 +23,15 @@
 #define	Y_VAL	(1 << 2)
 
 
+struct mode {
+	const char *name;	/* NULL for end of table */
+	int xres, yres;
+	int clkdiv;		/* pixel clock = 336 MHz/(clkdiv+1) */
+	int line_cycles;	/* 31.77 us for official VGA */
+	int hsync_end;		/* 0.79+3.77 us for official VGA */
+};
+
+
 void *map(off_t addr, size_t size);