[MIPS] Undefine scr_writew and scr_readw in <asm/vga.h>.

    
This is gluing the build of cirrusfb but really the mess that would need
cleaning and fixing is <video/vga.h> and <linux/vt_buffer.h> ...
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>