[coreboot] [patch] Fix CONFIG_GFXUMA in mtrr.c

[Myles Watson]

> The code between #if and #endif is only about UMA mode. The
> CONFIG_GFXUMA should be 1.
> We have another mode called side port mode. It is When the CONFIG_GFXUMA
> is 0.
> 
> Signed-off-by: Zheng Bao <zheng.bao at amd.com>
Acked-by: Myles Watson <mylesgw at gmail.com>

Thanks,
Myles