MIPS: Fix VGA_MAP_MEM macro.

Use the CKSEG1ADDR macro when calculating VGA_MAP_MEM.

[ralf@linux-mips.org: Include <asm/addrspace.h for CKSEG1ADDR.]

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5814/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>